A ct150 grade steel for sour service coiled tubing and method of manufacture

Abstract

The application discloses a CT150-grade acid-service continuous tubing steel and a manufacturing method thereof. By precisely controlling the content of C (0.19-0.21%) as a main strengthening element, the use amount of valuable alloy elements such as Ni, Nb, Mo and V can be appropriately reduced, and a refined collaborative manufacturing process including a'slag-remaining + double-slag' deep dephosphorization, low-temperature superheating continuous casting and dynamic soft reduction, four-stage slab heating, controlled rolling and controlled cooling and quenching and tempering is combined. The yield strength is 1050-1150 MPa, the tensile strength is 1250-1350 MPa, the yield strength ratio is less than or equal to 0.85, the elongation A after fracture is greater than or equal to 14%, the Charpy V-type notch impact energy KV2 at -40 DEG C is greater than or equal to 150 J, and the Rockwell hardness is less than or equal to 38 HRC. The steel has very high strength, good plasticity, deformation capacity and low-temperature toughness, can be safely used under a complex stress state in a deep well and brittle fracture can be avoided.

Description

Technical Field

[0001] This invention belongs to the field of metal materials technology, specifically relating to an ultra-high strength coiled tubing steel and its manufacturing method, and more particularly to a CT150 grade (yield strength ≥1050MPa) acidic service coiled tubing steel and its manufacturing method suitable for deep well and ultra-deep well operating environments containing acidic corrosive media such as hydrogen sulfide. Background Technology

[0002] As global oil and gas exploration and development continues to advance into deeper, ultra-deep, and high-pressure formations, the depth and pressure challenges faced by coiled tubing operations are becoming increasingly severe. In wells thousands or even tens of thousands of meters deep, coiled tubing must withstand enormous combined loads of tension, internal pressure, external extrusion, and cyclic bending. Therefore, the tubing steel must possess extremely high strength to prevent deformation and rupture. At the same time, many deep oil and gas reservoirs are rich in corrosive gases such as hydrogen sulfide (H2S) and carbon dioxide (CO2), which, in water-bearing environments, can easily cause severe corrosion problems in steel, such as hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), greatly threatening operational safety and tubing life.

[0003] Currently, most publicly available steel technologies for coiled tubing operate at strength levels of CT80-C100 (yield strength 550-700 MPa). For example, Chinese patent CN200710168545.3 discloses a high-ductility coiled tubing steel with a yield strength of 520-587 MPa. Chinese patent CN202210770237.2 discloses a coiled tubing steel with a tensile strength of 1000 MPa, but its yield strength is 786-818 MPa, which still cannot meet the requirements for higher strength levels. A common problem with existing technologies is the difficulty in consistently increasing material strength to a yield strength of over 1050 MPa (i.e., CT150 grade) while ensuring excellent low-temperature toughness, a low yield-to-strength ratio, and superior resistance to acidic corrosion. Even with the addition of large amounts of expensive alloying elements, some technologies show limited and fluctuating strength improvements, failing to achieve a good balance between high strength, high toughness, high corrosion resistance, and economy. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a CT150 grade acidic service coiled tubing steel and its manufacturing method, which effectively reduces the amount of expensive alloying elements while ensuring performance, and achieves a balance between cost and performance.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a CT150 grade steel for continuous oil tubing in acidic service, wherein the chemical composition of the steel, by weight percentage, comprises: C: 0.19-0.21%, Si: 0.15-0.25%, Mn: 1.05-1.15%, P ≤ 0.012%, S ≤ 0.0012%, Alt: 0.020-0.040%, Cu: 0.22-0.28%, Ni: 0.17-0.23%, Nb: 0.014-0.024%, Ti: 0.008-0.020%, Cr: 0.52-0.58%, Mo: 0.28-0.32%, V: 0.040-0.050%, B ≤ 0.0005%, Ca: 0.0015-0.0035%, N ≤ 0.0050%, with the balance being Fe and unavoidable inclusions.

[0006] This invention relates to a steel that "replaces gold with carbon," meaning that by precisely controlling the carbon content (0.19-0.21%), strength is primarily enhanced through carbon solid solution strengthening. This allows for a suitable reduction in the amount of expensive alloying elements such as Ni, Nb, Mo, and V added, achieving cost reduction and efficiency improvement. Too low a carbon content necessitates the addition of more alloying elements to maintain strength, increasing costs; too high a carbon content severely compromises the material's plasticity, toughness, and weldability. This invention eliminates the potential negative impacts of increased carbon content through a complete set of refined manufacturing processes.

[0007] The roles of each alloying element are briefly described as follows: C is the core element that ensures strength; Mn and Si play a solid solution strengthening role, and Mn can also improve hardenability; Cu, Ni, Cr, and Mo synergistically improve the corrosion resistance of steel in H2S-containing environments, among which Cu can also slow down hydrogen permeation, and Ni can counteract the hot brittleness of Cu; Nb, Ti, and V further improve strength and refine the microstructure through grain refinement and precipitation strengthening; Alt is used for deoxidation and grain refinement; the extremely low P, S, and B content aims to ensure high purity, prevent grain boundary embrittlement, and improve toughness and resistance to environmental cracking; Ca treatment is used to improve the morphology of inclusions and improve the purity of steel.

[0008] The mechanical properties of the CT150 grade steel for acidic service coiled tubing of this invention must meet the following requirements: yield strength of 1050-1150 MPa, tensile strength of 1250-1350 MPa, yield-to-tensile ratio ≤0.85, elongation after fracture A ≥14%, Charpy V-notch impact energy KV2 ≥150 J at -40℃, and Rockwell hardness ≤38 HRC. This combination of comprehensive properties ensures high safety, long service life, and good machinability of the material under complex stress and acidic corrosion environments in deep wells.

[0009] Secondly, the present invention provides a method for manufacturing the above-mentioned CT150 grade acidic service coiled tubing steel, which includes sequential steps of steelmaking, continuous casting, slab heating, rolling, cooling and coiling and heat treatment. The key lies in the coordinated control of the parameters of each process.

[0010] The steelmaking steps include: KR molten iron desulfurization: Ensures that the sulfur content S in the molten iron after desulfurization is ≤ 0.001%, laying the foundation for obtaining low-sulfur molten steel in the future.

[0011] Converter smelting: An economical and efficient deep dephosphorization process of "slag retention + double slag" is adopted. Specifically, a portion of the high-basicity slag from the previous heat is retained, and when the current heat reaches 25% of the total oxygen blowing progress, a portion of the high-phosphorus slag is discarded (double slag), and slag is re-formed. This process can significantly improve dephosphorization efficiency and reduce lime consumption by 30-40%. It is necessary to precisely control the steel temperature T1 at 1530-1540℃ and the carbon content C1 at 0.15-0.25wt% during the middle stage (85% progress) of the blowing process; at the end of the blowing process, the steel temperature T2 is controlled at 1580-1590℃ and the carbon content C2 at 0.040-0.060wt%, and the final slag basicity R (CaO / SiO2) is controlled at 3.0-3.8.

[0012] Refining and Vacuum Treatment: Molten steel undergoes deep desulfurization successively through an argon blowing station and an LF furnace, followed by RH vacuum circulation degassing treatment to significantly reduce the content of gases (H, N) and inclusions in the steel, resulting in high-purity molten steel. After vacuum treatment, 0.80-0.90 kg / ts of CaSi wire is fed in for calcium treatment to modify the inclusions.

[0013] The present invention employs an economical and efficient dephosphorization method in converter smelting: namely, slag retention and double slag dephosphorization, which can obtain extremely low P content in molten steel; the molten steel undergoes deep desulfurization treatment in a ladle refining furnace to obtain extremely low S content; the molten steel undergoes circulation degassing and slag removal treatment in a vacuum furnace, with strict control of circulation time and vacuum degree, to obtain extremely low N content and high purity molten steel.

[0014] Furthermore, the continuous casting step is crucial for controlling the internal quality of the billet and reducing segregation, including: A mold flux with specific properties was used (basicity 1.2-1.3, viscosity at 1300℃ 0.05-0.15 Pa·S, melting point 1070-1130℃), and cooling water parameters were optimized. Additionally, the mold taper was 1.1-1.2%, the cooling water flow rate on the wide side of the mold was 3100-3200 L / min, and the cooling water flow rate on the narrow side of the mold was 570-580 L / min. Weak cooling was used in the secondary cooling water of the continuous casting process to reduce the temperature rise of the billet surface, decrease the temperature gradient across the billet cross section, promote equiaxed crystal growth, inhibit excessive columnar crystal development, and reduce center segregation. Uniform secondary cooling was ensured to avoid stress concentration and cracking caused by localized strong cooling.

[0015] Controlling the thickness of the continuously cast billet to 225-235 mm is crucial, and also challenging, as controlling billet segregation within this thickness range is key. It is essential to maintain a low superheat, as high superheat leads to the development of columnar crystals, which easily form bridging and cause center segregation. Therefore, this invention controls the continuous casting superheat at 10-20℃ and the tundish temperature at 1518-1528℃; this promotes the formation and expansion of equiaxed crystal zones, weakens columnar crystal growth, and reduces columnar crystal bridging. However, if the superheat is too low, the tundish temperature will approach the liquidus temperature of the molten steel, potentially causing an interruption in the solidification process during casting.

[0016] During continuous casting, the billet pulling speed should be 0.9-1.1 m / min, maintaining a stable and moderate pulling speed. Excessively high pulling speeds will exacerbate solute enrichment at the solidification front, increasing the risk of center segregation; excessively low pulling speeds will affect production efficiency. Frequent adjustments to the pulling speed should be avoided to prevent instability at the solidification front, which could lead to increased segregation.

[0017] Applying a dynamic light pressure of 3.5-4.5 mm at the end of the billet solidification process compensates for the voids caused by solidification shrinkage in the billet core, preventing the molten steel enriched with solute from flowing back to the center, thereby directly reducing center segregation and center porosity.

[0018] Combined with billet surface temperature control, the billet surface temperature is controlled in segments according to different casting flow lengths, as shown in Table 1: Table 1

[0019] Furthermore, the slab heating employs a four-stage heating regime with slow temperature increases: the hot slab cooled to 160-180℃ is fed into the heating furnace and sequentially undergoes preheating (65-70 min, to 650-720℃), first heating (45-50 min, to 950-1050℃), second heating (50-60 min, to 1180-1240℃), and homogenization (40-50 min, to 1240-1300℃). This process facilitates homogenization of the slab's microstructure, ensures complete element solution, and prevents excessive thermal stress.

[0020] Furthermore, the rolling and cooling coiling steps include: Rough rolling ends at 1050-1080℃; it is carried out in two stages. The first stage of rough rolling adopts one pass of large deformation rolling with a reduction rate of 22-26% per pass. The second stage of rough rolling adopts seven passes of large deformation rolling with a reduction rate of more than 16% per pass, and the cumulative reduction rate of rough rolling is 78-82%. Water is sprayed on the surface for cooling during rough rolling to adjust the cooling gradient from the surface to the core of the billet, thereby reducing the segregation and coarse structure in the core.

[0021] Finish rolling is carried out in the non-recrystallized austenite region, with an end temperature of 870-920℃ and a cumulative reduction rate of 82-87%, achieving sufficient grain refinement.

[0022] After rolling, the material is accelerated to a final cooling temperature of 670-710℃ at a rate of 15-30℃ / s, and then wound at a winding temperature of 630-670℃.

[0023] The billet is heated in four stages in a hot rolling furnace. The heating time and temperature are strictly controlled. The temperature, reduction rate, cooling and coiling processes of rough rolling and finish rolling are also strictly controlled, which improves the strength of the coiled tubing steel and extends its service life.

[0024] Finally, the heat treatment step is quenching and tempering (quenching + tempering): Quenching: The hot-rolled and pickled steel strip is held at 930-980℃ for 10-20 minutes to allow carbon and alloying elements to fully dissolve in the austenite, and then water-quenched to obtain a uniform martensitic structure.

[0025] Tempering: High-temperature tempering is performed at 500-600℃ for 10-20 minutes to transform martensite into tempered sorbite with excellent strength and toughness, completely eliminating quenching stress and obtaining the final mechanical properties.

[0026] The reason this invention controls the quenching temperature at 930-980℃ and the quenching holding time at 10-20 minutes is that, under this process, carbon and alloying elements (Mn, Cr, Mo, etc.) can fully dissolve in austenite to form a uniform solid solution, laying the foundation for obtaining a martensitic structure through subsequent quenching. Excessively high temperatures or excessively long holding times will cause grain coarsening, leading to a significant decrease in the material's toughness, plasticity, and fatigue strength, and an increase in brittleness. Conversely, excessively low temperatures or excessively short holding times will result in undissolved carbides remaining, reducing quenching hardness and wear resistance.

[0027] The reason this invention controls the tempering temperature at 500-600℃ and the tempering holding time at 10-20 minutes is that the martensite structure after quenching is in a high-stress state and is very brittle. Tempering at 500-600℃ is a high-temperature tempering (quenching and tempering treatment), which promotes the decomposition of martensite into tempered sorbite (fine-grained cementite + ferrite), significantly reducing brittleness, improving plasticity and toughness, and preventing brittle fracture during downhole operations. The holding time of 10-20 minutes is to ensure sufficient transformation of the core and surface structure; too short a time will lead to insufficient tempering, while too long a time may cause excessive softening or grain coarsening.

[0028] This invention relates to CT150 grade (yield strength ≥1050MPa) steel for acidic service coiled tubing and its manufacturing method. Its core lies in the "carbon-for-gold" composition design, which precisely controls the C content (0.19-0.21%) as the main strengthening element, thereby appropriately reducing the amount of precious alloying elements such as Ni, Nb, Mo, and V. This is combined with a refined and collaborative manufacturing process including "slag retention + double slag" deep dephosphorization, low-superheat continuous casting and dynamic light reduction, four-stage slab heating, controlled rolling and cooling, and quenching and tempering treatment.

[0029] The core objective of this compositional and process co-design is to achieve ultra-high strength while ensuring that the steel possesses excellent toughness, a low yield strength ratio, and outstanding resistance to environmental cracking in deep well environments with acidic corrosive media such as H2S, high pressure, and high stress cycles. Strength is primarily enhanced through carbon solid solution strengthening, reducing costs; extremely low P and S content and calcium treatment ensure high steel purity, improving toughness and resistance to hydrogen-induced cracking (HIC); the synergistic effect of elements such as Cu, Ni, Cr, and Mo improves resistance to hydrogen sulfide stress corrosion cracking (SSCC). The subsequent entire manufacturing process aims to fully realize the potential of this compositional design from multiple levels, including smelting purity, billet uniformity, microstructure refinement, and toughening, while eliminating the potential negative impacts of increased carbon content on plasticity and toughness.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. A balance between ultra-high strength and excellent comprehensive mechanical properties has been achieved. CT150 grade coiled tubing steel has been successfully manufactured, with the following mechanical properties: yield strength of 1050-1150 MPa, tensile strength of 1250-1350 MPa, yield-to-tensile ratio ≤0.85, elongation after fracture A ≥14%, Charpy V-notch impact energy KV2 ≥150 J at -40℃, and Rockwell hardness ≤38 HRC. This means that the material not only has extremely high strength but also good plasticity, deformation capacity, and low-temperature toughness, enabling safe use under complex stress conditions in deep wells and avoiding brittle fracture.

[0031] 2. Cost optimization was achieved while ensuring high performance. The "carbon-for-gold" design concept reduces reliance on expensive alloying elements such as Ni, Mo, Nb, and V by precisely controlling the carbon content. This allows for effective control of raw material costs while meeting CT150-grade performance specifications, thereby enhancing the product's market competitiveness.

[0032] 3. Significantly improved durability and service life in acidic corrosive environments. Extremely low sulfur and phosphorus content and calcium treatment improve inclusion morphology, enhance steel purity, and strengthen resistance to HIC. The addition of elements such as Cu, Ni, Cr, and Mo synergistically improves resistance to SSCC. Experiments show that this steel grade can withstand over 120 cycles under simulated harsh conditions, far exceeding the 20 cycles of ordinary steel grades, greatly extending the service life of coiled tubing in hydrogen sulfide-containing oil and gas wells.

[0033] 4. Stability and reproducibility of performance are ensured through coordinated control of the entire process. From deep dephosphorization and desulfurization in the converter, to controlling center segregation through low superheat and dynamic light pressure in continuous casting, to slow heating of the slab to reduce internal stress, to controlled rolling and cooling to refine grains, and finally to tempering treatment to obtain tempered sorbite microstructure, each process parameter is carefully designed and coupled. This systematic control ensures uniform microstructure and stable performance in the final product, reduces performance fluctuations, and guarantees material reliability from the very source of manufacturing. Attached Figure Description

[0034] Figure 1 This is a microstructure diagram of the tempered steel from Example 3. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in more detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0036] The components, processes, and performance of the embodiments of the present invention are shown in Tables 2-9, respectively.

[0037] As can be seen from Table 2-9, the present invention provides a CT150 grade steel for acidic service coiled tubing and its manufacturing method, specifically with a yield strength of 1050-1150MPa, a tensile strength of 1250-1350MPa, a yield ratio ≤0.85, an elongation after fracture A ≥14%, an impact energy KV2 ≥150J at -40℃, and a Rockwell hardness ≤38.

[0038] The above embodiments are merely best examples and are not intended to limit the implementation of the present invention.

[0039] Table 2. List of component values ​​(wt, %) for each embodiment and comparative example of the present invention Table 3. List of main steelmaking process parameters for each embodiment and comparative example of the present invention. Table 4. List of main continuous casting process parameters for each embodiment and comparative example of the present invention. Table 5. Surface temperature of billet corresponding to continuous casting flow length in various embodiments and comparative examples of the present invention. Table 6. List of main slab heating process parameters for each embodiment and comparative example of the present invention. Table 7. List of main rolling process parameters for each embodiment and comparative example of the present invention. Table 8. List of main heat treatment process parameters for each embodiment and comparative example of the present invention. According to the national standards GB / T 228.1 "Metallic materials - tensile testing at room temperature" and GB / T 230.1-2018 "Metallic materials - Rockwell hardness test", the performance parameters of the steels in each embodiment and comparative example were tested, and the test results are shown in Table 9.

[0040] Table 9. Statistical Table of Main Performance Tests for Various Embodiments and Comparative Examples of the Invention The performance test results shown in Table 9 indicate that all Examples 1-10, manufactured using the composition design and manufacturing method of this invention, exhibit yield strengths between 1065-1140 MPa and tensile strengths between 1278-1348 MPa, meeting and exceeding the performance requirements of CT150 grade (≥1050 MPa). Furthermore, their yield-to-tensile strength ratios are all no higher than 0.85, and their impact energy at -40℃ exceeds 150 J, demonstrating excellent comprehensive performance characterized by high strength, high toughness, and a low yield-to-tensile strength ratio. In contrast, Comparative Examples 1 and 2, due to deviations in composition and / or process from the scope of this invention, cannot achieve the CT150 grade strength and have relatively lower toughness. In particular, Examples 1, 4, and 7, etc., exceeded 120 cycles in the durability test simulating harsh acidic environments, significantly outperforming the ordinary steel grades mentioned in the comparative examples and background art (approximately 20 cycles). This fully verifies the balance between high strength, high corrosion resistance, and long service life achieved by this invention through the synergistic process of the entire process under the 'carbon-for-gold' design concept.

[0041] The CT150 grade coiled tubing steel prepared by this invention operates in oil wells at depths of over 10,000 meters, where the operating environment is complex. The steel tubing undergoes bending and torsion, bearing multi-directional loads and significant internal pressure. It is exposed to various chemical substances, such as strong acids and high-pressure fluids, from extremely cold environments to high-temperature oil and gas layers, and is subjected to repeated stresses. Even under such complex working conditions, the steel grade of this invention can still be used more than 120 times, while other existing coiled tubing steels generally only last for about 20 times.

[0042] In summary, this invention provides CT150 grade steel for coiled tubing, meeting the requirements of a yield strength of 1050-1150 MPa, tensile strength of 1250-1350 MPa, yield-to-tensile ratio ≤0.85, elongation after fracture A ≥14%, impact energy KV2 ≥150 J at -40℃, and Rockwell hardness ≤38. While maintaining high strength, it also ensures toughness and impact resistance, possesses good machinability, reduces the risk of brittle fracture under complex working conditions, has a high resistance to various external forces (tensile, compressive, bending, etc.), and a long service life. It is suitable for use in harsh environments such as complex terrain, high temperature, and high pressure, meeting the needs of coiled tubing under different working conditions.

[0043] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A CT150 grade steel for continuous oil tubing in acidic service, characterized in that, The chemical composition of the steel, by weight percentage, includes: C: 0.19-0.21%, Si: 0.15-0.25%, Mn: 1.05-1.15%, P ≤ 0.012%, S ≤ 0.0012%, Alt: 0.020-0.040%, Cu: 0.22-0.28%, Ni: 0.17-0.23%, Nb: 0.014-0.024%, Ti: 0.008-0.020%, Cr: 0.52-0.58%, Mo: 0.28-0.32%, V: 0.040-0.050%, B ≤ 0.0005%, Ca: 0.0015-0.0035%, N ≤ 0.0050%, with the balance being Fe and unavoidable inclusions.

2. The CT150 grade acidic service coiled tubing steel according to claim 1, characterized in that, The mechanical properties of the steel meet the following requirements: yield strength of 1050-1150MPa, tensile strength of 1250-1350MPa, yield ratio ≤0.85, elongation after fracture A ≥14%, Charpy V-notch impact energy KV2 ≥150J at -40℃, and Rockwell hardness ≤38HRC.

3. A method for manufacturing CT150 grade acidic service coiled tubing steel according to claim 1 or 2, characterized in that, The process includes the sequential steps of steelmaking, continuous casting, slab heating, rolling, cooling and coiling, and heat treatment; the steelmaking steps include: The molten iron is desulfurized by KR to ensure that the sulfur content S in the desulfurized molten iron is ≤ 0.001%; Converter smelting: The slag retention and double-slag dephosphorization method is adopted. Slag is dumped when the total oxygen blowing progress reaches 25%. When the total oxygen blowing progress reaches 85%, the steel temperature T1 is controlled at 1530-1540℃ and the carbon content C1 is 0.15-0.25wt%. When the blowing reaches the end point, the steel temperature T2 is controlled at 1580-1590℃ and the carbon content C2 is 0.040-0.060wt%. The final slag basicity R is 3.0-3.

8. Argon blowing, LF refining and deep desulfurization, and RH vacuum treatment are performed in sequence. After the vacuum treatment, calcium treatment is carried out.

4. The manufacturing method according to claim 3, characterized in that, The continuous casting step includes: The casting process uses a medium-carbon steel protective slag with high basicity, low viscosity, and high melting point. The basicity of the protective slag is 1.2-1.3, the viscosity at 1300℃ is 0.05-0.15 Pa·S, and the melting point is 1070-1130℃. The superheat of continuous casting is controlled at 10-20℃, the tundish temperature at 1518-1528℃, the billet pulling speed at 0.9-1.1 m / min, and the thickness of the continuous casting billet at 225-235mm. Apply a dynamic light pressure of 3.5-4.5 mm to the solidification end region of the billet; The surface temperature of the billet is controlled in segments according to different casting flow lengths, as shown in Table 1: Table 1 。 5. The manufacturing method according to claim 3, characterized in that, The slab heating process is as follows: the continuously cast slab cooled to 160-180℃ is fed into the heating furnace and subjected to four stages in sequence: preheating, first heating, second heating, and homogenization. The preheating stage lasts for 65-70 minutes, with the slab temperature at the end of the stage being 650-720℃; the first heating stage lasts for 45-50 minutes, with the slab temperature at the end of the stage being 950-1050℃; the second heating stage lasts for 50-60 minutes, with the slab temperature at the end of the stage being 1180-1240℃; and the homogenization stage lasts for 40-50 minutes, with the slab temperature at the end of the stage being 1240-1300℃.

6. The manufacturing method according to claim 3, characterized in that, The rolling, cooling, and coiling steps include: The heated slab is rough rolled at a finishing temperature of 1050-1080℃, with a cumulative reduction rate of 78-82%. Next, finish rolling is performed in the non-recrystallized austenite region. The finish rolling temperature is 870-920℃, and the cumulative reduction rate is 82-87%. After rolling, the steel is cooled to a final cooling temperature of 670-710℃ at a cooling rate of 15-30℃ / s, and then coiled at a coiling temperature of 630-670℃ to obtain a hot-rolled steel coil.

7. The manufacturing method according to claim 3, characterized in that, The heat treatment step is a tempering process, including: Quenching: Hold the steel strip at 930-980℃ for 10-20 minutes, and then quench it with water as the medium; Tempering: Hold at 500-600℃ for 10-20 minutes.

8. The manufacturing method according to claim 4, characterized in that, The taper of the mold during the continuous casting process is 1.1-1.2%, the cooling water flow rate of the wide side of the mold is 3100-3200 L / min, and the cooling water flow rate of the narrow side of the mold is 570-580 L / min.

9. The manufacturing method according to claim 6, characterized in that, The roughing process is carried out in two stages. The first stage of roughing uses one pass of large deformation rolling with a reduction rate of 22-26%. The second stage of roughing uses seven passes of large deformation rolling with a reduction rate of more than 16% per pass.

10. The manufacturing method according to claim 6, characterized in that, Water is sprayed onto the surface for cooling during the rough rolling process.

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