A hot rolling production method for high-strength, high-toughness, medium-thickness X70 pipeline steel strip

Abstract

This invention provides a hot continuous rolling production method for high-strength and high-toughness medium-thickness X70 pipeline steel strip, belonging to the field of pipeline steel strip production technology. The method includes: hot metal pretreatment, converter smelting, LF ladle refining, RH vacuum refining, slab continuous casting, slab heating, reciprocating rough rolling, CVC finishing rolling, multi-stage laminar flow cooling, and coiling. This invention, through a rationally designed alloy composition, the addition of an appropriate amount of Ce element, and the purification of steel using an LF-RH dual refining process, employs a low-temperature fast rolling and large reduction process to refine grains during hot rolling. Furthermore, in the cooling stage, a segmented cooling method combining front-end concentrated cooling, rapid cooling, air cooling, and fine-tuning is used to control phase transformation. Therefore, the X70 pipeline steel strip produced by this invention has a uniform and dense acicular ferrite structure, possessing high strength, high elongation, and good low-temperature impact toughness and drop hammer tear resistance, thus meeting the requirements of oil and gas transportation pipelines for thick-gauge pipeline steel.

Description

Technical Field

[0001] This invention belongs to the technical field of pipeline steel strip production process, specifically relating to a hot continuous rolling production method for high-strength and tough medium-thickness X70 pipeline steel strip. Background Technology

[0002] Currently, pipeline transportation has become the most important and economical mode of transportation for long-distance oil and natural gas transport. In order to increase pipeline capacity and reduce costs, high-grade steel, large-diameter, thick-walled, high-pressure, and high-capacity pipeline materials have become the inevitable development direction of pipeline transportation. At present, the actual application of pipeline engineering at home and abroad is still mainly based on X70 and X80 grade pipeline steel. Among them, X70 pipeline steel, as one of the mainstream representative grades of current oil and gas main pipelines, is widely used in the laying of long-distance, high-capacity oil and natural gas main pipelines due to its good comprehensive performance.

[0003] Because pipelines need to operate stably for a long time under complex geographical and climatic conditions, this places high demands on the comprehensive performance of pipeline materials. Therefore, the development of its technology has always revolved around a core objective: how to develop X70 pipeline steel with good comprehensive performance while meeting engineering requirements such as high pressure, large capacity, large diameter, and large wall thickness, so as to achieve a synergistic improvement in high strength and high toughness and provide a reliable guarantee for the safe and stable transportation of oil and gas energy.

[0004] The strength and toughness engineering application indicators of pipeline steel include both the general requirements specified in the basic standards and the differentiated technical indicators for different service scenarios. Its core indicators such as mechanical properties, impact toughness and drop hammer performance together constitute the technical benchmark for evaluating the comprehensive performance of pipeline steel. To achieve the above key performance indicators, it is necessary to design and control the entire process from alloy composition system, smelting process and rolling process to achieve a good balance of strength and toughness in pipeline steel.

[0005] Upon searching, the following three patent applications belong to the same technical field as this invention and solve similar technical problems. Therefore, the three are used as comparative documents below to illustrate several technical defects of the above-mentioned technical solutions.

[0006] 1. Patent application CN111270156A discloses a "production process for hot-rolled X70 pipeline steel with a thickness of 20.0-25.4mm". The alloy composition, by weight percentage, is: C 0.03-0.12%, Si≤0.45%, Mn 1.50-1.70%, Cr≤0.25%, Nb+V+Ti≤0.15%, P≤0.025%, S≤0.015%, Als≤0.05%, N≤0.010%, Ca≤0.005%, with the remainder being Fe and unavoidable impurities. The laminar flow cooling system in this patent uses forced cooling technology to reduce the cooling water temperature and increase the post-rolling cooling rate, thereby improving the strength and toughness of the pipeline steel. However, this technical solution has the following shortcomings: First, its alloy system does not involve the addition of rare earth elements, so it cannot give full play to the beneficial effects of rare earth elements in purifying steel and refining grains; Second, its cooling process is relatively simple, only reducing the cooling water temperature to increase the cooling rate, which limits its ability to finely control the phase transformation of the microstructure and makes it difficult to obtain a uniform and fine acicular ferrite microstructure; Third, its product thickness specifications are 20.0-25.4mm, and the applicability of 16mm medium-thickness products is not explained.

[0007] 2. Patent application CN105803323A discloses a "method for producing X70 pipeline steel plates using a relaxation cooling process." The alloy composition, by weight percentage, is: C 0.02–0.07%, Si 0.20–0.40%, Mn 1.20–1.80%, Cr 0.10–0.30%, Nb≤0.06%, Ti≤0.02%, P≤0.015%, S≤0.010%, with the remainder being Fe and unavoidable impurities. This patent employs a relaxation cooling process, where the steel is relaxed on the roller table for 20–60 seconds after roughing and finishing rolling, and then enters an ultra-fast cooling device to cool to 560–480°C. The above technical solution has the following shortcomings: First, it mainly relies on an ultra-fast cooling system, which is not available on all hot continuous rolling production lines. First, it is highly dependent on equipment; second, its final cooling temperature is relatively high, ranging from 560 to 480°C. Final cooling at such high temperatures is not conducive to the formation of low-temperature phase transformation structures such as acicular ferrite or bainite, which usually require lower temperatures to fully form. Excessively high final cooling temperatures will result in the final microstructure containing more proeutectoid ferrite or pearlite, thereby reducing the strength and toughness of the material; third, it does not provide specific low-temperature impact toughness and drop hammer tear performance indicators for the X70 pipeline steel it produces.

[0008] 3. Patent application CN102653836B discloses a "production method of X70 pipeline steel hot-rolled coil". Its alloy composition, by weight percentage, is: C≤0.07%, Si≤0.30%, Mn≤1.60%, Nb≤0.07%, Ti0.010~0.025%, Cr≤0.30%, with the remainder being Fe and unavoidable impurity elements. This patent achieves a high-strength, high-toughness fine-grained acicular ferrite structure. It employs a cooling process of post-rolling ultra-fast cooling + laminar flow cooling pre-section cooling + laminar flow cooling post-section fine cooling shielding. After finishing rolling on a precision continuous rolling mill, rapid cooling and laminar flow cooling are performed, with rapid cooling and laminar flow cooling controlled separately. The technical solution of cooling the steel plate in the cold section by 80-100℃ and the steel plate in the front section of the cooling system by 80-100℃ has the following shortcomings: First, its cooling process is relatively complex, involving multiple stages such as rapid cooling, front section cooling, and rear section fine cooling shielding, requiring precise control of multiple cooling ranges, which is difficult to control stably on a conventional production line; Second, the temperature of the steel plate is below 620℃ when it reaches the middle section of laminar flow cooling, and the control requirements for this intermediate temperature are strict, increasing the difficulty of process control; Third, it does not provide specific strength and toughness indicators for the X70 pipeline steel, especially the data on low-temperature impact toughness and drop hammer tear performance, making it impossible to assess the reliability of its products in low-temperature service environments.

[0009] In summary, existing technologies still have room for improvement in alloy composition design, rolling process control, and especially the fine-tuning of cooling paths. Specifically, existing technologies suffer from technical defects such as the lack of rare earth element optimization of the alloy system, the simplistic or overly complex cooling process that is difficult to control, or the lack of sufficient performance data support. Therefore, how to stably produce medium-thickness X70 pipeline steel strips with ultra-fine grains and excellent strength-toughness matching through full-process optimization remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0010] This invention provides a hot continuous rolling production method for high-strength and tough medium-thickness X70 pipeline steel strip, aiming to solve the problem that the comprehensive performance of X70 pipeline steel is difficult to further improve in the existing technology.

[0011] This invention is based on an integrated smelting, casting and rolling process of BOF converter smelting, LF ladle refining, RH vacuum refining, CC slab continuous casting and conventional hot continuous rolling. It adopts a multi-element microalloying composition design concept, rationally matches microalloying elements, and adds a certain amount of Ce element on this basis. Combined with specific hot continuous rolling and controlled cooling processes, it obtains X70 pipeline steel with ultrafine grains and a high strength and toughness match.

[0012] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0013] A hot continuous rolling production method for high-strength, high-toughness, medium-thickness X70 pipeline steel strip includes the following steps: The smelting process includes, in sequence, molten iron pretreatment, converter smelting, LF ladle refining, and RH vacuum refining.

[0014] In the hot metal pretreatment step, the blast furnace hot metal is desulfurized, desiliconized, and dephosphorized to provide qualified hot metal for subsequent converter smelting.

[0015] In the converter smelting process, a BOF converter is used for smelting, with bottom blowing of argon throughout the process. Specifically, the purpose of bottom blowing of argon is to homogenize the composition and temperature of the molten steel and promote the flotation of inclusions. The final stage of converter smelting controls the composition and temperature of the molten steel to provide favorable conditions for subsequent refining.

[0016] In the LF ladle refining process, the molten steel entering the LF station undergoes desulfurization and white slag formation. The white slag has strong reducing and adsorption capabilities, effectively removing sulfur and oxygen from the molten steel. Specifically, the white slag formation process involves adding slag-forming materials such as lime and fluorite, controlling the slag's alkalinity and fluidity, and making the slag appear white or light gray. The formation of white slag indicates that the iron oxide content in the slag has been reduced to a low level, which is beneficial for the deep purification of the molten steel.

[0017] In the RH vacuum refining process, vacuum circulation degassing is performed. The principle of RH vacuum refining is to use a vacuum chamber to lift and circulate the molten steel, allowing the gas in the molten steel to escape under vacuum conditions. Specifically, the lifting gas flow rate of this invention is controlled in three stages: 50~70 m³ / h in the early stage, 70~90 m³ / h in the middle stage, and 80~100 m³ / h in the later stage.

[0018] The technical effects of adopting a phased increase in gas flow rate are as follows: the first stage uses a lower gas flow rate to avoid slag splashing and equipment damage caused by the initial violent boiling; the second stage appropriately increases the gas flow rate to enhance the circulation degassing effect; the third stage further increases the gas flow rate to fully remove residual hydrogen, nitrogen and other gases in the molten steel and promote the flotation of inclusions.

[0019] The vacuum circulation time is 15-25 minutes. The clean blowing time before calcium treatment is greater than 3 minutes. The purpose of calcium treatment is to modify Al2O3 inclusions, converting high-melting-point Al2O3 into low-melting-point calcium aluminate, preventing nozzle blockage, and improving the pourability of molten steel. The clean blowing time of more than 3 minutes ensures the uniformity of steel composition and temperature before calcium treatment.

[0020] The continuous casting process involves continuously casting the molten steel obtained from the smelting process to obtain a billet. Specifically, the continuous casting of slabs adopts full-process protective pouring to prevent secondary oxidation of the molten steel due to contact with air during the pouring process. The argon flow rate of the protective sleeve is 5~20m³ / h, forming an argon protective atmosphere at the connection between the protective sleeve and the long nozzle to prevent air from entering. The crystallizer casting speed is stably controlled at 1.0~1.3m / min. Stable control of the casting speed is conducive to uniform billet shell growth, while too fast a casting speed may lead to an excessively thin billet shell and the risk of steel leakage, while too slow a casting speed will reduce production efficiency.

[0021] The fluctuation of the liquid level in the crystallizer should be controlled within ±5mm. If the fluctuation is too large, the protective slag will be drawn into the molten steel to form inclusions, and the billet shell thickness will also be uneven.

[0022] The mold flux used is either a special mold flux for pipeline steel or a special mold flux for low-carbon low-alloy steel. The special mold flux for pipeline steel has a suitable melting temperature and viscosity, which can fully lubricate the surface of the billet and absorb the floating inclusions.

[0023] The secondary cooling water system adopts a special cooling mode for pipeline steel. Based on the phase transformation characteristics of the steel grade and the temperature field distribution of the billet, the water spray volume of each cooling section is reasonably allocated to control the uniformity of billet cooling.

[0024] The process employs dynamic light reduction with a reduction amount of 3-5 mm. The principle of dynamic light reduction is to apply a certain amount of reduction at the end of the solidification of the billet to compensate for the solidification shrinkage volume and reduce center segregation and center porosity. However, if the reduction amount is too small, the effect will be insignificant, and if the reduction amount is too large, it may lead to internal cracks.

[0025] The billet thickness is 230~250mm. The alloy composition of the billet by weight percentage is: C not more than 0.06%, Si not more than 0.40%, Mn 1.40~1.70%, P not more than 0.012%, S not more than 0.003%, Mo not more than 0.15%, Nb not more than 0.065%, Ti not more than 0.030%, Ni not more than 0.15%, Ce 0.002~0.006%, Als 0.015~0.045%, N not more than 0.005%, O not more than 0.002%, H not more than 0.0015%, with the balance being Fe and unavoidable trace elements.

[0026] In the above alloy composition design, the role of each element and the basis for determining its content range are as follows: Carbon (C) is an important matrix strengthening element. In high-grade pipeline steels of X70 and above, the C content is generally controlled below 0.06% to promote the formation of acicular ferrite in the room temperature microstructure. Acicular ferrite is a substructure with high dislocation density, which can provide both high strength and good toughness. At the same time, the low C content ensures that the pipeline steel has good impact and weldability. If the C content is too high, it will lead to an increase in pearlite content, reduce low-temperature toughness, and exacerbate the hardening and embrittlement tendency of the weld heat-affected zone. If the C content is too low, the strength will be insufficient and it will be difficult to meet the strength requirements of X70 steel grade.

[0027] Mn primarily plays a role in solid solution strengthening and controls phase transformation behavior. Mn can lower the temperature of the austenite-to-ferrite phase transformation, thereby refining ferrite grains. Mn can also promote the nucleation of acicular ferrite, which is beneficial for obtaining a room temperature microstructure dominated by acicular ferrite. The strength of pipeline steel increases with the increase of Mn content. However, excessive Mn content can easily cause segregation bands, forming Mn-rich regions in the center of the slab, resulting in uneven microstructure and properties after rolling. In addition, Mn and S form MnS inclusions, which are elongated into strips during rolling and are prone to hydrogen-induced crack initiation during post-rolling cooling. Therefore, the Mn content in this invention is controlled at 1.40~1.70%.

[0028] The Nb, V, and Ti composite microalloying is the key technical feature of this invention. Nb, V, and Ti are strong carbonitride forming elements and can form fine carbonitride precipitates with C and N. These precipitates play multiple roles in the hot working process: First, during the slab heating stage, undissolved Nb and Ti carbonitrides pin the austenite grain boundaries, inhibiting austenite grain coarsening. Second, during the rough rolling stage, deformation-induced carbonitride precipitation promotes dynamic recrystallization of austenite, refining the austenite grains. Third, during the finish rolling stage, strain-induced carbonitride precipitation strongly inhibits austenite recrystallization, flattening the austenite grains and increasing the grain boundary area per unit volume. Finally, during the cooling and coiling stages, the fine carbonitride precipitates provide precipitation strengthening.

[0029] By adding Nb, V, and Ti in combination, the combined effects of fine grain strengthening, precipitation strengthening, solid solution strengthening, and dislocation strengthening can be fully utilized, significantly improving the overall performance of pipeline steel. Specifically, in this invention, Nb is controlled at no more than 0.065%, Ti is controlled at no more than 0.030%, and the content of N and C is controlled to form the best precipitation strengthening effect.

[0030] Mo is an essential alloying element in high-grade pipeline steel, and its functions are as follows: First, Mo shifts the pearlite transformation curve to the right, thus delaying the pearlite transformation and increasing the temperature of the maximum pearlite transformation rate. Second, Mo lowers the temperature of the maximum bainite transformation rate, clearly separating the C-curves of pearlite and bainite transformations. This results in stronger kinetics for bainite transformation under the same cooling conditions, making it easier to obtain a low-temperature phase transformation microstructure dominated by bainite or acicular ferrite. Third, Mo can suppress proeutectoid ferrite. The formation of proeutectoid ferrite is a soft phase, which reduces the strength of the material; fourth, Mo can promote the formation of acicular ferrite with high-density dislocation substructure or fine-structured low-carbon bainite; fifth, Mo can also increase the solid solubility product of Nb in austenite, promote the solid solution of Nb in austenite, and thus play a better precipitation strengthening role in the later cooling process. However, since Mo is a precious metal and the alloy cost is very high, it should be added as little as possible while ensuring that it must be added. Therefore, in this invention, Mo is controlled to be no more than 0.15%.

[0031] Ni and Cu are added as auxiliary alloying elements. Ni is the main alloying element for austenite formation and stabilization, which can expand the austenite phase region and lower the austenitizing temperature. At the same time, Ni can improve the low-temperature toughness and fracture toughness of pipeline steel. In addition, Ni can also improve the weather resistance and corrosion resistance of pipeline steel. Cu, as a non-carbide forming element, can also expand the austenite phase region and has certain solid solution strengthening and precipitation strengthening effects. Specifically, in this invention, Ni is controlled to be no more than 0.15%.

[0032] Ce is a unique additive element in this invention. By adding a certain amount of rare earth Ce to the above alloy composition, the beneficial effects of rare earth elements in purifying steel and refining grains are realized. Because rare earth Ce has a strong affinity for oxygen and sulfur in steel, it can form high-melting-point rare earth oxides and rare earth sulfides. These rare earth inclusions are spherical, small in size, and uniformly distributed, unlike MnS which is elongated during rolling. At the same time, rare earth inclusions can act as nuclei for heterogeneous ferrite formation, promoting grain refinement. In addition, rare earth elements can also change the morphology and distribution of inclusions, improving the isotropy of steel. In this invention, the Ce content is controlled at 0.002~0.006%. If the Ce content is too low, the purification effect will be insignificant; if it is too high, coarse rare earth inclusions may be formed, which may impair performance.

[0033] Als is added as a deoxidizer, controlled at 0.015~0.045% to ensure sufficient deoxidation of molten steel; P, S, N, O and H are all harmful impurity elements, controlled at P not exceeding 0.012%, S not exceeding 0.003%, N not exceeding 0.005%, O not exceeding 0.002% and H not exceeding 0.0015% respectively to ensure the purity of steel.

[0034] In the heating process, the billet is fed into a walking beam furnace for heating and homogenization; the slab loading methods include hot-charged direct rolling and cold-charged hot-charged loading, wherein: Hot-charge direct rolling refers to the direct feeding of high-temperature slabs after continuous casting into a heating furnace, utilizing the residual heat of the slabs to save energy. Specifically, when loading the furnace using the hot-charge direct rolling method, the furnace time is 180-240 minutes, and the soaking time is 30-55 minutes.

[0035] Cold charging and hot delivery refers to loading the billet into the furnace for heating after it has cooled to room temperature. This method is suitable for situations where the billet needs to be inspected or ground. Specifically, when loading the billet into the furnace using the cold charging and hot delivery method, the time spent in the furnace is 180 to 270 minutes, and the soaking time is 35 to 60 minutes.

[0036] The furnace time includes a heating section and a soaking section; wherein: the heating section is used to rapidly raise the temperature of the slab to the target temperature, and the soaking section is used to make the temperature inside and outside the slab uniform; specifically, the heating temperature is 1080~1140℃, and the soaking temperature is 1170~1230℃.

[0037] The furnace exit temperature is 1170~1210℃, based on the target furnace exit temperature.

[0038] The temperature difference between the core and surface of the slab should be controlled within 20℃.

[0039] The aforementioned suitable heating temperature ensures complete austenitization of ferrite grains and sufficient solid solution of microalloying elements such as Nb and Ti, while also preventing the slab from overheating in the furnace and causing excessive growth of the original austenite grains. In addition, excessive growth of the original austenite grains will be inherited by the room temperature phase transformation structure, resulting in coarsening of the final structure and impairing strength and toughness. Controlling the core-surface temperature difference within 20°C ensures the uniformity of the structure along the thickness direction of the slab.

[0040] In the rough rolling process, the heated billet is subjected to reciprocating multiple passes on a double stand to obtain an intermediate billet. After exiting the heating furnace, the billet is descaled by high-pressure water jet. High-pressure water descaling uses a high-pressure water jet to impact the surface of the billet and remove the iron oxide scale formed during the heating process. If the iron oxide scale is not removed, it will be pressed into the surface of the steel plate during the rolling process, forming surface defects.

[0041] After descaling, the slab enters the roughing mill for rolling. Roughing is carried out in the austenite recrystallization zone, where the temperature is relatively high, usually above 1000℃, and austenite has good plasticity. After large reduction deformation, the austenite undergoes complete dynamic recrystallization. Specifically, dynamic recrystallization refers to the simultaneous nucleation and growth of austenite grains during hot deformation. Through repeated recrystallization, the original coarse cast grains are broken into fine equiaxed grains, and the grain size can be refined to about 5 micrometers.

[0042] The roughing rolling process employs a three-pass reciprocating rolling process: R1R2, R2R1, and R1R2. This means the slab passes back and forth between two mills, R1 and R2, with each pass completing one pass. This three-pass reciprocating rolling process ensures sufficient cumulative deformation while avoiding excessive rolling force exceeding limits that could result from excessive reduction in a single pass. Specifically, the cumulative reduction in the R1 mill is greater than 80.0 mm, and the cumulative reduction rate is greater than 50%; the cumulative reduction in the R2 mill is greater than 89.0 mm, and the cumulative reduction rate is greater than 60%.

[0043] Cumulative reduction and reduction rate are key parameters to ensure sufficient dynamic recrystallization of austenite. The thickness of the intermediate slab is 55~65mm; if the intermediate slab is too thick, it will increase the finishing rolling load, and if it is too thin, the slab temperature will drop too quickly, which is not conducive to the finishing rolling temperature control; the final rolling temperature of the roughing mill is 1020~1120℃, which ensures that the roughing mill is completed in the austenite recrystallization zone and provides a suitable entry temperature for the finishing mill.

[0044] In the finishing rolling process, the intermediate billet is continuously finished rolled on multiple stands to obtain hot-rolled steel strip. After rough rolling, the slab is briefly warmed on the waiting roller table. Specifically, the warming time is 10 to 25 seconds. The purpose of the warming is to make the temperature of the slab surface and core more uniform and to appropriately reduce the temperature to enter the non-recrystallized austenite region. If the warming time is too short, the temperature drop will be insufficient; if the warming time is too long, the temperature drop will be too large, leading to rolling difficulties.

[0045] After the temperature is reached, the slab enters the flying shear, which is used to cut off the head and remove the irregularly shaped parts of the head to ensure smooth threading. Then, it undergoes a high-pressure water descaling process to remove the iron oxide scale generated in the second stage before finishing rolling. After that, it enters the finishing mill for rolling.

[0046] Specifically, the finishing mill inlet temperature is 960~1000℃, and a six-stand four-high CVC mill is used for continuous rolling, that is, continuous variable crown control. The axial movement of the rolls is used to achieve flexible control of the plate shape. The six-stand arrangement ensures sufficient cumulative reduction, which can reduce the intermediate billet of about 60mm to the finished product thickness of 16mm.

[0047] The reduction rate distribution of each stand from F1 to F6 meets the following requirements: the total reduction rate of stands from F1 to F2 is greater than 50%, and the cumulative reduction rate of stands from F3 to F6 is greater than 60%. That is, the first two stands undertake the main reduction task, rapidly thinning the intermediate billet, while using the deformation heat generated by the large reduction to compensate for the temperature drop; the last four stands further thin the billet and control the final plate shape and dimensional accuracy.

[0048] The final rolling temperature of the steel from the finishing mill is 790~830℃.

[0049] The production of high-grade pipeline steel adopts a low-temperature fast rolling process with large reduction, that is, at a lower final rolling temperature, a faster rolling speed and a larger reduction per pass.

[0050] Finishing rolling is carried out in the temperature-controlled rolling zone of the non-recrystallized austenite region. The temperature in this stage is relatively low, usually below 950°C, and the austenite no longer recrystallizes. Rolling in the non-recrystallized austenite region has the following effects: First, the flattened original austenite grains are continuously elongated and flattened under the plastic deformation force of rolling, and the grains change from equiaxed to flattened. Second, high-density deformation twins and deformation bands are generated inside the grains. These deformation defects are high-energy regions and become preferential nucleation sites for ferrite phase transformation. Third, at the same time, microalloying elements such as Nb and Ti undergo strain-induced precipitation. Strain-induced precipitation refers to the increase in the solid solubility of microalloying elements in austenite under deformation, which preferentially precipitates extremely fine carbonitride particles at defects such as grain boundaries and subgrain boundaries. The precipitated particles further pin the grain boundaries, inhibit recrystallization, and serve as ferrite nucleation nuclei. The above effects together promote ferrite phase transformation and ferrite nucleation, and promote the refinement of ferrite grains during subsequent cooling phase transformation.

[0051] In addition, as strain accumulates during hot rolling, deformation-induced ferrite phase transformation and dynamic recrystallization of ferrite can occur. Deformation-induced ferrite phase transformation refers to the ferrite phase transformation that occurs directly during deformation in the non-recrystallized austenite region without the need for cooling. Specifically, dynamic recrystallization of ferrite refers to the recrystallization of newly generated ferrite grains during deformation, which further refines the grains and ultimately yields an ultrafine grain structure with a size of about 1.5 micrometers.

[0052] The cooling process refers to the multi-stage laminar flow cooling of hot-rolled steel strip. The cooling process used in this invention is designed for a multi-stage integrated laminar flow cooling system, which includes a first dense cooling zone, a rough cooling zone, a second dense cooling zone, and a fine cooling zone arranged in sequence. Specifically, the first dense cooling zone includes 5 sets of manifolds, the rough cooling zone includes 10 sets of manifolds, the second dense cooling zone includes 3 sets of manifolds, and the fine cooling zone includes 2 sets of manifolds.

[0053] Among them, the dense cooling zone refers to the area with a high density of manifolds, which can provide stronger cooling capacity; the coarse cooling zone is a conventional laminar flow cooling area with moderate cooling capacity; and the fine cooling zone is used for temperature fine-tuning and has weaker cooling capacity.

[0054] The aforementioned cooling system has multiple cooling modes, including dedicated cooling for high-strength steel and front-end cooling, enabling process-oriented control of the cooling process.

[0055] The cooling process used in this invention consists of front-end concentrated cooling, rapid cooling, air cooling, and fine-tuning. The purpose and effect of each stage will be explained in detail below.

[0056] In the initial rapid cooling stage, the manifold opening mode of the dense cooling zone is 3.5 / 5, meaning that 3.5 out of every 5 manifolds in the first dense cooling zone are open, equivalent to a 70% opening ratio; both upper and lower manifolds are fully open, meaning that the cooling water for the upper and lower surfaces of the steel strip is turned on simultaneously; the cooling water flow rate is 7500~8500 m³ / h. The purpose of this stage is to cool the steel strip from the final rolling temperature of approximately 810℃ to the first-stage final cooling temperature at the fastest possible cooling rate. Rapid cooling can suppress the formation of proeutectoid ferrite and pearlite, because proeutectoid ferrite and pearlite require a relatively long diffusion time. Rapid cooling allows the steel strip to quickly pass through the proeutectoid ferrite and pearlite phase transformation zone and enter the acicular ferrite and bainite phase transformation zone. At the same time, rapid cooling can also refine the microstructure after phase transformation, because the faster the cooling rate, the less time the ferrite nuclei have to grow after formation, resulting in a finer grain size.

[0057] In the air-cooling stage, after passing through the first dense cooling zone and the rough cooling zone, the steel strip is air-cooled to the target temperature range. Air cooling refers to exposing the steel strip to air for natural cooling without applying cooling water. The purpose of the air-cooling stage is twofold: first, to homogenize the temperature of the steel strip, as the rapid cooling in the preceding stage may have resulted in temperature unevenness along the width or thickness direction; the air-cooling stage uses heat conduction to homogenize the temperature. Second, to avoid excessive internal stress caused by over-cooling. The air-cooling stage allows the steel strip to complete part of the phase transformation under relatively mild conditions, which is beneficial for microstructure uniformity and reducing internal stress.

[0058] In the fine-tuning stage, the final temperature is adjusted to the target winding temperature through the fine cooling zone. The fine cooling zone has a weak cooling capacity and a small number of manifolds, which allows for precise control of the cooling amount. The purpose of fine-tuning is to compensate for any temperature deviations that may occur during the air cooling stage and to accurately hit the winding temperature window of 400±20℃. Precise control of the winding temperature is crucial for the final microstructure. Excessive deviation can lead to changes in the microstructure and affect performance stability.

[0059] The final cooling temperature of the first stage is 480~520℃. This temperature is the end of the rapid cooling phase and the start of air cooling. The basis for selecting this temperature range is as follows: If the final cooling temperature of the first stage is too high, above 550℃, the steel strip may have undergone partial pearlite phase transformation, which is not conducive to the formation of acicular ferrite in the subsequent stage; if the final cooling temperature of the first stage is too low, below 450℃, the temperature of the steel strip is too low, and it is difficult to ensure that the subsequent coiling temperature is within the range of 380~420℃. At the same time, the excessively low temperature may lead to the formation of martensite, which will damage the toughness.

[0060] The coiling process involves coiling the cooled hot-rolled steel strip. High-grade pipeline steel typically employs a low-temperature coiling process, with a coiling temperature of 380~420℃. The purpose of low-temperature coiling is to control the ferrite phase transformation, which is conducive to the formation of low-temperature phase transformation structures such as acicular ferrite or bainite. Within the temperature range of 380~420℃, the phase transformation driving force is large, resulting in fine acicular ferrite grains and dispersed carbide distribution, thus achieving good strength and toughness. If the coiling temperature is too high, above 450℃, coarse pearlite or blocky ferrite is easily formed, reducing strength and toughness. If the coiling temperature is too low, below 350℃, hard phases in martensite or bainite may form, reducing toughness and weldability. A high-strength coiling machine is used for coiling. A high-strength coiling machine refers to a large coiling machine with high coiling tension and capacity, capable of withstanding the high tension of high-strength steel strips, ensuring neat coil shape, and covering steel strips with thicknesses of 16~26mm.

[0061] The X70 pipeline steel strip produced by the above method has a typical fine-grained acicular ferrite microstructure. The typical characteristics of acicular ferrite are that the grains are needle-shaped or lath-shaped, interwoven with each other, and contain a high dislocation density. The grain size reaches grade 12.7, which belongs to the ultrafine grain category. The microstructure is uniform and dense, and the cross-sectional microstructure has good uniformity. That is, from the surface of the steel plate to the center, the grain size and microstructure type change very little. The inclusions do not exceed grade 0.5, and the banded structure does not exceed grade 0.5, indicating that the steel is pure and the compositional segregation is slight.

[0062] The beneficial effects of this invention are as follows: 1. This invention, through reasonable alloy composition design, adds an appropriate amount of Ce element and combines it with LF-RH double refining process, effectively purifies the steel, reduces non-metallic inclusions, and improves the purity of the steel; compared with the prior art, the content of impurity elements such as sulfur, phosphorus, oxygen, nitrogen, and hydrogen in this invention is more strictly controlled, ensuring the low-temperature toughness and resistance to hydrogen-induced cracking of pipeline steel. 2. In this invention, the hot rolling process employs a two-stand reciprocating roughing and a six-stand CVC finishing, combined with a low-temperature fast rolling process with large reduction. Deformation control is implemented in both the austenite recrystallization and non-recrystallization regions, refining the austenite grains and providing more nucleation sites for ferrite phase transformation, thereby obtaining an ultrafine grain structure. Compared with the prior art, the rolling process of this invention is more refined, ensuring sufficient recrystallization through three-pass reciprocating roughing and promoting deformation-induced phase transformation through large reduction in finishing. 3. In the cooling stage, this invention utilizes a multi-stage laminar flow cooling system, implementing a stepped cooling path consisting of rapid, intense cooling in the front section, air cooling, and fine-tuning, precisely controlling the phase transformation process. Specifically, the rapid, intense cooling in the front section provides sufficient supercooling to promote the nucleation of acicular ferrite; the air cooling section in the middle section makes the temperature of the steel strip tend to be uniform to avoid excessive stress; and the fine-tuning in the final section ensures accurate targeting of the low-temperature coiling temperature window. Compared with the prior art, the cooling process of this invention not only ensures a sufficient cooling rate but also avoids the problem of complex cooling processes being difficult to control. 4. The X70 pipeline steel strip prepared by the method of the present invention has a uniform and fine acicular ferrite structure, which can combine high strength, high elongation, good low temperature impact toughness and drop hammer tear performance, and can meet the engineering requirements of medium and thick X70 pipeline steel for oil and gas transmission pipelines. Attached Figure Description

[0063] Figure 1 Metallographic images of the X70 pipeline steel strip stretched in the 45° direction produced in Embodiments 1 and 2 of this invention; Figure 2 Metallographic diagrams of the transverse tensile plates of X70 pipeline steel strips produced in Embodiments 1 and 2 of the present invention.

[0064] In the picture: a1 and a2 represent the core structure of the plate in Examples 1 and 2, respectively; b1 and b2 represent the tissue at one-quarter of the plate in Examples 1 and 2, respectively; c1 and c2 represent the edge structure of the plate in Examples 1 and 2, respectively. Detailed Implementation

[0065] The present invention will be further described in detail below with reference to specific embodiments. The embodiments of the present invention are not limited to the following examples; various changes made without departing from the spirit of the present invention are all within the scope of protection of the present invention.

[0066] In a specific embodiment of the present invention, the smelting process is first performed: After hot metal pretreatment, BOF converter is used for smelting. Argon is blown into the bottom throughout the converter smelting process, and the bottom blowing flow rate is controlled at 50~150m³ / h according to the smelting stage to homogenize the composition and temperature of the molten steel. The final temperature of the converter is 1650~1680℃, and the final carbon content is controlled at 0.03~0.05%. Deoxidizers and alloys are added during the tapping process for pre-deoxidation and preliminary alloying.

[0067] Subsequently, LF ladle refining is carried out. In LF ladle refining, after the molten steel enters the station, the slag system is first adjusted by adding lime and fluorite to control the slag basicity at 3.5~4.5. Electricity is applied to raise the temperature and deoxidation and desulfurization operations are carried out to produce white slag. The characteristics of white slag are that the slag surface is white or light gray, indicating that the FeO content in the slag is less than 0.5%. Specifically, the refining time is controlled at 30~50 minutes to ensure uniform composition and stable temperature.

[0068] Subsequently, RH vacuum refining is carried out. The vacuum degree of RH vacuum refining is controlled below 100Pa. The gas flow rate is controlled in three stages: 60m³ / h in the early stage, 80m³ / h in the middle stage, and 90m³ / h in the later stage. The vacuum circulation time is 18~22 minutes. For calcium treatment, silicon-calcium wire or calcium-iron wire is used. The wire feeding speed is controlled at 2~4m / s, and the wire feeding amount is 1.5~2.5m / t steel. The clean blowing time before calcium treatment is greater than 3 minutes, and the soft blowing time after calcium treatment is greater than 8 minutes.

[0069] Then, the continuous casting process is carried out; the slab continuous casting adopts full-process protective pouring, and the argon flow rate of the protective sleeve is 5~20m³ / h, which is adjusted according to the pouring time and the erosion of the sleeve; the crystallizer speed is stably controlled at 1.1~1.2m / min. The liquid level fluctuation in the crystallizer is controlled within ±3mm, and an electromagnetic braking or automatic liquid level control system is used to ensure the stability of the liquid level; specifically, the protective slag of the crystallizer adopts pipeline steel special protective slag, and its main physical and chemical properties are: melting point 1100~1150℃, viscosity 0.8~1.2Pa·s; the secondary cooling water adopts pipeline steel special cooling mode, and the specific water volume is controlled at 0.8~1.0L / kg steel; the full-process dynamic light reduction is adopted, with a reduction of 4mm, and light reduction is carried out at the end of the solidification of the slab; the slab thickness is 230mm and the cross-sectional width is 1500mm; after the slab is removed from the line, it is inspected for quality, and qualified slabs are sent for hot rolling.

[0070] The alloy composition of the billet, by weight percentage, is: C 0.05%, Si 0.245%, Mn 1.62%, P 0.0080%, S 0.0015%, Mo 0.11%, Nb 0.050%, Ti 0.018%, Ni 0.095%, Ce 0.003%, Als 0.030%, N 0.0046%, O 0.0013%, H 0.0001%, with the balance being Fe and unavoidable trace elements.

[0071] Two specific examples are given below to illustrate the hot continuous rolling process and its product performance in detail.

[0072] Example 1 The slabs coming off the continuous casting machine are fed into a walking beam furnace for heating and homogenization via hot conveying. The temperature of the slabs after continuous casting is 800~900℃. They are then fed directly into the furnace via a heat-insulating roller conveyor and hot conveying direct rolling is adopted. The slabs are in the furnace for 180~240 minutes and homogenization time is 30~55 minutes.

[0073] The heating furnace is divided into a preheating section, a heating section, and a soaking section. Specifically, the temperature of the preheating section is 800~1000℃, the temperature of the heating section is 1080~1140℃, the temperature of the soaking section is 1170~1230℃, and the furnace exit temperature is 1190℃. The temperature of the slab is measured after it exits the furnace. Steel is tapped after the target temperature is reached. The temperature difference between the core and the surface of the slab is ensured by controlling the heating and soaking time. The actual measured temperature difference between the core and the surface does not exceed 20℃.

[0074] After exiting the furnace, the slab undergoes high-pressure water descaling at a pressure of 20-25 MPa. The nozzle angle and spacing are optimized to ensure complete surface coverage of the slab. After descaling, the iron oxide scale on the slab surface is completely removed, leaving a silvery-gray appearance. The slab then enters the roughing mill, which is a two-stand tandem arrangement. Both R1 and R2 are four-high reversible mills. Roughing is performed using three reciprocating passes: R1R2, R2R1, and R1R2. The initial rolling temperature is 1048℃. Specifically, the pass pressure of the R1 mill... The reduction amounts were 29.8mm, 27.9mm, and 25.3mm respectively, with pass reduction rates of 12.7%, 20.9%, and 24% respectively; the pass reduction amounts for the R2 mill were 35.3mm, 35.3mm, and 19.3mm respectively, with pass reduction rates of 17.3%, 20.9%, and 24% respectively; the intermediate slab thickness was 60mm, and the final rolling temperature for the roughing mill was 1044℃; during the roughing milling process, the slab width naturally widened, with a final width of 1538mm.

[0075] After rough rolling, the slab is placed on a warming roller table for 15-20 seconds. During the warming period, the surface temperature of the slab decreases while the core temperature remains basically unchanged, and the temperature gradually becomes uniform. After warming, the slab enters the flying shear, which cuts off the irregular parts at the head and tail, with a cut length of 200-300 mm. Then, a high-pressure water descaling is performed, with the descaling pressure slightly higher than that of the rough descaling, at 25-30 MPa.

[0076] The mill then proceeds to the finishing mill, a six-stand, four-high CVC mill, numbered F1 to F6. The finishing mill inlet temperature is 980℃, and the reduction rate and exit thickness for each stand are as follows: F1 has an inlet thickness of 60 mm and an outlet thickness of 44.2 mm, with a reduction rate of 26.5%; F2 has an outlet thickness of 32.5 mm; F3 has an outlet thickness of 25.5 mm; F4 has an outlet thickness of 20.6 mm; F5 has an outlet thickness of 18.0 mm; and F6 has an outlet thickness of 16.0 mm. The total reduction rate of F1 to F2 is 53.0%, and the total reduction rate of F3 to F6 is 64.7%. The finishing rolling temperature is 820℃, and the outlet speed of F6 is 1.5 m / s.

[0077] The rolling process employs automatic thickness control and automatic shape control systems to ensure the thickness accuracy and shape quality of the finished product.

[0078] After finishing rolling, laminar flow cooling is carried out using a multi-stage laminar flow cooling system, which is arranged as follows: 5 sets of manifolds in the first dense cooling zone, 10 sets of manifolds in the roughing cooling zone; 3 sets of manifolds in the second dense cooling zone, and 2 sets of manifolds in the finishing cooling zone; each set of manifolds includes an upper manifold and a lower manifold, which spray water onto the upper and lower surfaces of the steel strip respectively; the cooling water is circulating water, and the water temperature is controlled at 25~35℃.

[0079] The cooling process consists of initial rapid cooling, air cooling, and fine-tuning. Specifically, in the initial rapid cooling step, the manifold opening mode of the dense cooling zone is 3.5 / 5, meaning that in the first dense cooling zone, the first three manifolds are fully open, the fourth is half open, and the fifth is closed, with both upper and lower manifolds fully open. The cooling water flow rate is 8000 m³ / h, and the steel strip passes through the first dense cooling zone and the rough cooling zone at a speed of approximately 1.5 m / s. The final cooling temperature of the first stage is 420℃, which is controlled by feedback from a pyrometer, and the water flow rate is adjusted according to the measured temperature. After passing through the second dense cooling zone, the steel strip enters the air cooling zone, with an air cooling distance of 10-20 meters. After air cooling, the steel strip enters the fine cooling zone, where the number of manifolds and the water flow rate are adjusted according to the target temperature, and the coiling temperature is 400℃.

[0080] The coiling process uses a high-power coiler, specifically an underground coiler with a drum diameter of 760mm. The coiling tension is controlled at 15~25kN / m, while a reasonable coiling speed ratio is maintained, with a coiling speed of 1.5~2.0m / s, matching the exit speed of the finishing mill. After coiling, the steel coil is sent to the cooling zone for natural cooling, and then inspected, packaged, and stored.

[0081] As shown in Tables 1-3, comprehensive performance tests were conducted on the 16mm thick X70 pipeline steel plates produced in the above specific embodiments: Its tensile yield strength in the 45° direction is 578 MPa, its tensile strength is 642 MPa, its yield ratio is 0.90, and its elongation is 22.0%. Its transverse tensile yield strength is 615 MPa, tensile strength is 677 MPa, yield ratio is 0.91, elongation is 17.5%, and A50 elongation is 35.8%. The Charpy V-impact and drop hammer tear tests were conducted at temperatures of -15℃, -25℃, -40℃, -60℃, and -80℃, respectively. No ductile-brittle transition occurred at the low temperature of -80℃, and the average impact toughness was 193J. The impact energy of the three parallel specimens were 166J, 216J, and 198J, respectively. The fiber cross-sectional area of ​​the drop hammer tear was 100% / 100% at each test temperature.

[0082] Table 1 Chemical composition of X70 steel plate (wt.%) Table 2 Roughing Rolling Procedure of Example 1 Table 3. Finishing Rolling Procedure of Example 1 like Figure 1 and 2 As shown, the microstructure of the steel plate is typical acicular ferrite with a large number of dispersed carbides distributed in the matrix; the grain size is grade 12.7, the microstructure is uniform and fine, and the cross-sectional microstructure has good uniformity; the non-metallic inclusions of type A, B, C, and D are all no more than grade 0.5; the banded microstructure is grade 0.5; and the Vickers hardness HV10 is 216.

[0083] Example 2 The slabs coming off the continuous casting machine are also fed into a walking beam furnace for heating and homogenization using a hot delivery method. The heating process is basically the same as in Example 1. The slabs are in the furnace for 180 to 240 minutes, the homogenization time is 30 to 55 minutes, and the furnace exit temperature is 1190°C.

[0084] After exiting the furnace, the slab undergoes high-pressure water descaling, and then enters the roughing mill for rolling. The roughing mill employs the same three-pass reciprocating rolling as in Example 1, but the reduction in each pass is slightly different: The roughing rolling temperature is 1045℃; The pass reductions of the R1 mill were 29.8 mm, 28.1 mm, and 25.2 mm, respectively, and the pass reduction rates were 12.7%, 21.0%, and 23.9%, respectively. The pass reductions of the R2 mill were 35.3 mm, 35.3 mm, and 19.2 mm, respectively, and the pass reduction rates were 17.3%, 20.9%, and 23.9%, respectively. The intermediate billet thickness is 60 mm; the final rolling temperature of the roughing mill is 1040℃.

[0085] After rough rolling, the slab is left to warm on the roller table for 15-20 seconds. After warming, the slab enters the flying shear head, then undergoes fine descaling, and then enters the finishing mill for rolling. The entry temperature of the finishing mill is 970-990℃, and the reduction ratio distribution of each stand is slightly different from that in Example 1. The total reduction rate for F1~F2 is 52.5%, the total reduction rate for F3~F6 is 65.1%, the finishing rolling temperature is 815℃, and the exit speed for F6 is 1.5m / s.

[0086] After finishing rolling, laminar flow cooling is performed using the same multi-stage laminar flow cooling system and cooling process as in Example 1: the final cooling temperature of the first stage is 430°C, and the coiling temperature is 420°C. A high-strength coiler is also used for coiling.

[0087] As shown in Tables 4 and 5, the comprehensive performance of the 16mm thick X70 pipeline steel plate produced in Example 2 was tested. Its tensile yield strength in the 45° direction is 572 MPa, its tensile strength is 644 MPa, its yield ratio is 0.73, and its elongation is 22.5%. Its transverse tensile yield strength is 594 MPa, tensile strength is 660 MPa, yield ratio is 0.90, elongation is 18.0%, and A50 elongation is 38.7%. In the impact toughness test, no ductile-brittle transition occurred at a low temperature of -80℃, and the average impact toughness was 211J. The impact energy of the three parallel specimens were 218J, 207J and 209J, respectively. The fiber cross-sectional area of ​​the drop shear was 100% / 100% at each test temperature.

[0088] Table 4 Roughing Rolling Procedure of Example 2 Table 5. Finishing Rolling Procedure for Example 2 like Figure 1 and 2 As shown, the microstructure of the steel plate is typical acicular ferrite with a large number of dispersed carbides distributed in the matrix; the grain size is grade 12.7, the microstructure is uniform and fine, and the cross-sectional microstructure has good uniformity; inclusions do not exceed grade 0.5, and banded microstructure is grade 0.5. The Vickers hardness HV10 is 214.

[0089] Table 6 Hot Continuous Rolling Process Parameters Table 7 Tensile properties of steel plates (45° direction) Table 8 Tensile properties of steel plates (transverse tension) Table 9 Impact toughness (longitudinal) and drop weight performance (45° direction) of steel plates Table 10 Metallographic Testing of Steel Plates As shown in Tables 6-10, the test results of Examples 1 and 2 demonstrate that the X70 pipeline hot-rolled steel plate produced using the alloy composition and production process described in this invention not only possesses excellent comprehensive strength and toughness but also exhibits a uniform ultrafine grain structure and low inclusion level. In particular, its low-temperature impact toughness and drop hammer tear performance are significantly superior to existing technologies, thus well meeting the construction needs of oil and gas pipeline projects in cold regions.

Claims

1. A hot continuous rolling production method for high-strength, high-toughness, medium-thickness X70 pipeline steel strip, characterized in that, Includes the following steps: The smelting process includes hot metal pretreatment, converter smelting, LF ladle refining and RH vacuum refining; The continuous casting process involves continuously casting the molten steel obtained from the smelting process to obtain a billet. In the heating process, the billet is fed into a walking beam furnace for heating and homogenization; In the roughing process, the heated billet is subjected to a double-stand reciprocating multi-pass roughing process to obtain an intermediate billet. In the finishing rolling process, the intermediate billet is continuously finished rolled on multiple stands to obtain hot-rolled steel strip; The cooling process involves subjecting the hot-rolled steel strip to multi-stage laminar flow cooling. The coiling process involves coiling the cooled hot-rolled steel strip.

2. The hot continuous rolling production method for high-strength and tough medium-thickness X70 pipeline steel strip according to claim 1, characterized in that, In the aforementioned smelting process: Bottom-blown argon throughout the converter smelting process; LF ladle refining is used to desulfurize molten steel and produce white slag. In RH vacuum refining, the flow rate of the boosting gas is controlled in three stages: the first stage is 50~70 m³ / h, the second stage is 70~90 m³ / h, and the third stage is 80~100 m³ / h. The vacuum circulation time is 15-25 minutes; The air blowing time before calcium treatment should be greater than 3 minutes.

3. The hot continuous rolling production method for high-strength and tough medium-thickness X70 pipeline steel strip according to claim 1, characterized in that, In the continuous casting process: Full-process protective pouring is adopted; The argon flow rate for the protective sleeve is 5~20 m³ / h; The crystallizer pulling speed is 1.0~1.3m / min, the liquid level fluctuation in the crystallizer is controlled within ±5mm, and the protective slag for the crystallizer is special protective slag for pipeline steel or special protective slag for low carbon low alloy steel. The secondary cooling water system adopts a dedicated cooling mode for pipeline steel. The entire process is characterized by dynamic light pressing, with a pressing amount of 3~5mm.

4. The hot continuous rolling production method for high-strength, high-toughness, medium-thickness X70 pipeline steel strip according to claim 1, characterized in that, The alloy composition of the billet, by weight percentage, is as follows: C not greater than 0.06%, Si not greater than 0.40%, Mn 1.40~1.70%, P not greater than 0.012%, S not greater than 0.003%, Mo not greater than 0.15%, Nb not greater than 0.065%, Ti not greater than 0.030%, Ni not greater than 0.15%, Ce 0.002~0.006%, Als 0.015~0.045%, N not greater than 0.005%, O not greater than 0.002%, H not greater than 0.0015%, with the balance being Fe and unavoidable trace elements.

5. The hot continuous rolling production method for high-strength, high-toughness, medium-thickness X70 pipeline steel strip according to claim 1, characterized in that, In the heating process: The furnace exit temperature is 1170~1210℃, and the temperature difference between the core and surface of the slab is controlled within 20℃. When the billet is charged into the furnace using the hot-feeding direct rolling method, the furnace time is 180~240 minutes and the soaking time is 30~55 minutes. When the billet is charged into the furnace using the cold charging and hot delivery method, the time in the furnace is 180~270 minutes, the soaking time is 35~60 minutes, the heating temperature is 1080~1140℃, and the soaking temperature is 1170~1230℃.

6. The hot continuous rolling production method for high-strength, high-toughness, medium-thickness X70 pipeline steel strip according to claim 1, characterized in that, In the roughing process: After the steel is tapped from the heating furnace, it undergoes high-pressure water descaling. Rough rolling is carried out in the austenite recrystallization zone; The rolling process employs a dual-mill, three-pass reciprocating rolling method. The dual-mill system comprises an R1 mill and an R2 mill, with the three passes referring to sequential rolling of R1R2, R2R1, and R1R2. The cumulative pass reduction of the R1 mill is greater than 80.0 mm, and the cumulative pass reduction rate is greater than 50%. The cumulative pass reduction of the R2 mill is greater than 89.0 mm, and the cumulative pass reduction rate is greater than 60%. The thickness of the intermediate billet is 55~65mm; The roughing and finishing rolling temperatures are 1020~1120℃.

7. The hot continuous rolling production method for high-strength and high-toughness medium-thickness X70 pipeline steel strip according to claim 1, characterized in that, In the finishing rolling process: After rough rolling, the slab is heated on the roller table for 10-25 seconds, then subjected to high-pressure water descaling, and then enters the finishing mill. The inlet temperature of the finishing mill is 960~1000℃; The rolling process is carried out continuously using a six-stand four-high CVC finishing mill, wherein the six stands include stands F1 to F6, the total reduction rate of stands F1 to F2 is greater than 50%, and the cumulative reduction rate of stands F3 to F6 is greater than 60%. The finishing rolling temperature is 790~830℃.

8. The hot continuous rolling production method for high-strength and tough medium-thickness X70 pipeline steel strip according to claim 1, characterized in that, The cooling process adopts a multi-stage laminar flow cooling system, which includes a first dense cooling zone, a coarse cooling zone, a second dense cooling zone, and a fine cooling zone in sequence. The multi-stage laminar flow cooling system employs the following cooling processes in sequence: front-stage rapid cooling, air cooling, and fine-tuning. The front-end cooling system has a rapid cooling temperature of 480~520℃.

9. The hot continuous rolling production method for high-strength and high-toughness medium-thickness X70 pipeline steel strip according to claim 8, characterized in that, In the first dense cooling zone of the multi-stage laminar flow cooling system, 3.5 out of every 5 sets of manifolds are open, with both upper and lower manifolds fully open, and the cooling water volume is 7500~8500 m³ / h.

10. The hot continuous rolling production method for high-strength and tough medium-thickness X70 pipeline steel strip according to claim 1, characterized in that, In the winding process: The winding temperature is 380~420℃; A high-powered winding machine is used for winding, with a winding capacity covering steel strips with a thickness of 16~26mm.

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