A low-hardness l450m pipeline steel with high deformation resistance and a production method thereof

Abstract

This invention discloses a low-hardness L450M pipeline steel with resistance to large deformation and its production method. The chemical composition of the pipeline steel, by weight percentage, is as follows: C: 0.04-0.06%, Si: 0.15-0.35%, Mn: 1.35-1.65%, P≤0.002%, S≤0.002%, Cr: ≤0.1%, Ni: ≤0.1%, Cu: ≤0.20%, Mo: ≤0.1%, Nb: 0.035-0.055%, V: ≤0.1%, Ti: 0.008-0.020%, B: ≤0.0005%, Al: 0.020-0.040%, Ca: 0.001-0.004%, N≤0.002%, with the balance being Fe and unavoidable impurities. A dual-phase microstructure of quasi-polygonal ferrite + degenerate pearlite was prepared through LF+RH refining-continuous casting-controlled rolling and cooling, with a grain size ≥9, yield strength ≥460MPa, tensile strength ≥600MPa, uniform elongation ≥11%, and hardness ≤210HV10. It is suitable for harsh service scenarios prone to pipeline displacement, such as areas with frequent geological activity, permafrost zones, and earthquake-prone areas, and can meet the comprehensive performance requirements of long-distance oil and gas pipelines for materials with "high strength, low hardness, and high deformation resistance."

Description

Technical Field

[0001] This invention belongs to the field of high-performance pipeline steel manufacturing technology, specifically relating to a low-hardness L450M pipeline steel with resistance to large deformation and its production method. Background Technology

[0002] In recent years, global oil and gas resource development has been continuously expanding into remote and complex areas, requiring oil and gas pipelines to traverse special terrains such as permafrost, earthquake zones, and geological subsidence areas. The freeze-thaw cycles, ground displacement, and seismic activity in these regions easily subject pipelines to large deformation loads, including axial tension, lateral bending, and overall displacement. If the pipeline steel's resistance to large deformations is insufficient, it can lead to excessive localized plastic deformation, weld cracking, or even pipeline fracture, causing oil and gas leaks, environmental pollution, and major safety accidents. Therefore, resistance to large deformations has become a key indicator for high-grade pipeline steel, requiring materials to possess not only high yield strength but also excellent uniform elongation and a reasonable yield-to-tensile ratio to absorb external load energy through plastic deformation and avoid sudden failure.

[0003] Meanwhile, the pipe-making process for pipeline steel has strict requirements on material hardness. Whether it's the continuous bending and forming of spiral welded pipes or the pre-bending and joining processes of straight seam welded pipes, excessive hardness will hinder the plastic flow of the sheet metal, easily causing forming defects such as cold bending cracks and edge cracking. It will also exacerbate the wear of forming equipment and increase pipe-making costs. Currently, conventional pipeline steels resistant to large deformation often use a low-carbon, medium-manganese composition system combined with microalloying elements such as Nb and Ti to improve strength and deformation resistance. This is combined with controlled rolling and controlled cooling processes to refine grains and generate acicular ferrite or bainite structures. However, these processes are often accompanied by the formation of granular bainite, lath bainite, and M / A islands. Among them, M / A islands, due to their high proportion of martensite, significantly increase the hardness of the steel. Granular bainite has large differences in mechanical properties from the matrix structure, which not only leads to hardness fluctuations but also easily causes stress concentration during deformation, thus weakening the resistance to large deformation and creating a performance conflict between strength, hardness, and deformation resistance.

[0004] While some of the existing patented technologies can achieve low hardness or high strength, they are difficult to balance with resistance to large deformations. For example, the low-hardness, high-strength, high-grade pipeline steel disclosed in CN202010943003.4 controls the hardness to ≤210HV10, yield strength ≥460MPa, and tensile strength ≥550MPa through composition design and process optimization. However, its core focus is on the balance of strength and hardness under normal harsh environments, without involving the design of resistance to large deformation. The uniform elongation and yield strength ratio are difficult to adapt to large deformation conditions. The low yield strength ratio and low hardness subsea pipeline steel L485MO disclosed in CN202110407186.2 controls the hardness to 210-220HV10 and yield strength to 530-558MPa. Although the yield strength ratio is ≤0.85, this steel grade is designed for subsea oil and gas transportation, focusing on resistance to seawater corrosion and crack arrest performance, without considering the deformation resistance requirements under geological large deformation scenarios. Moreover, the combination of strength and elongation cannot meet the requirements for plastic deformation absorption under large deformation conditions. The low-hardness acid-resistant subsea pipeline steel X65MOS disclosed in CN202410772105.2 has a hardness of ≤195HV10. It focuses on the combined requirements of seabed acid corrosion resistance and explosion resistance, and its resistance to large deformation is not the design focus. Therefore, it cannot meet the usage requirements of geological large deformation conditions.

[0005] In summary, there is an urgent need to develop a pipeline steel technology that balances high strength, low hardness, and high deformation resistance. This technology should achieve a synergistic effect among these three aspects, ensuring a sufficient proportion of soft phases such as quasi-polygonal ferrite while enhancing resistance to large deformations through a rational strengthening mechanism. This would address the problems of excessively high hardness, difficulty in pipe forming, and poor matching between deformation resistance and hardness in existing pipeline steel technologies. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a low-hardness L450M pipeline steel with high deformation resistance and a production method thereof, in order to solve the technical problems of high hardness, difficulty in pipe forming, poor matching between deformation resistance and hardness, and high alloy cost of existing pipeline steel with high deformation resistance, and to achieve a comprehensive match of "high strength-low hardness-high deformation resistance".

[0007] The technical solution adopted by this invention to solve the above problems is as follows: a low-hardness L450M pipeline steel with resistance to large deformation, the chemical composition by mass percentage is: C: 0.04-0.06%, Si: 0.15-0.35%, Mn: 1.35-1.65%, P≤0.002%, S≤0.002%, Cr: ≤0.1%, Ni: ≤0.1%, Cu: ≤0.20%, Mo: ≤0.1%, Nb: 0.035-0.055%, V: ≤0.1%, Ti: 0.008-0.020%, B: ≤0.0005%, Al: 0.020-0.040%, Ca: 0.001-0.004%, N≤0.002%, with the balance being Fe and unavoidable impurities. The final thickness specification range of the steel plate (the thickness range that the product can cover, such as 6-30mm or more specifically).

[0008] The design principles of the above chemical components are as follows: C: 0.04-0.06%. C is the most basic strengthening element in steel, and this invention employs an extremely low C content design. Excessive C content not only reduces the plasticity, toughness, and weldability of steel, but also forms more carbides during cooling, increasing hardness and hindering the achievement of low hardness. An extremely low C content helps form a quasi-polygonal ferrite soft phase matrix, reducing hardness and improving uniform elongation.

[0009] Si: 0.15-0.35%. Si is a commonly used deoxidizing element with a certain solid solution strengthening effect. Too low a Si content results in insufficient deoxidation, while too high a content reduces the toughness and plasticity of the steel. This invention limits the range to 0.15-0.35% to ensure good deoxidation and moderate solid solution strengthening, while not being less than the effect on plasticity and toughness. Mn: 1.35-1.65%. Mn is an important solid solution strengthening element that can improve the strength of steel, expand the stable austenite phase region, and refine the grains. However, excessive Mn content can lead to segregation, increased hardenability, and increased hardness. This invention controls the Mn content at 1.35-1.65% to balance strength requirements and low hardness limitations.

[0010] P, S: P≤0.002%, S≤0.002%. P and S are harmful impurity elements in steel. P easily causes cold brittleness, and S easily forms MnS inclusions, reducing resistance to HIC. This invention adopts an ultra-low P and S design, controlling the content of both to within ≤0.002%, which helps to purify the steel, reduce microstructure segregation and inclusion content, and provide a pure steel foundation for obtaining uniform and fine quasi-polygonal ferrite and degenerate pearlite microstructure.

[0011] Cr, Ni, Mo, and Cu are all alloying elements that have a certain strengthening effect and improve hardenability, but they all increase the cost and hardness of the alloy. This invention strictly limits the content of each element (≤0.1%, ≤0.1%, ≤0.1%, ≤0.20%), preferably not adding them or only at residual levels, in order to control cost and hardness.

[0012] Nb: 0.035-0.055%. Nb forms Nb(C,N) carbonitrides in steel. These fine precipitates pin grain boundaries, inhibiting recrystallization and grain coarsening. Simultaneously, dynamic precipitation during rolling can pin dislocations, resulting in ultrafine deformable austenite. Excessive Nb content increases precipitation strengthening, leading to increased hardness; insufficient content results in inadequate grain refinement.

[0013] V: ≤0.1%. V is also a microalloying strengthening element. This invention limits the upper limit to 0.1% to control the strengthening effect and avoid an increase in hardness.

[0014] Ti: 0.008-0.020%. Ti forms fine TiN particles, which hinder grain growth and suppress recrystallization during high-temperature austenitization.

[0015] B: ≤0.0005%. The B content needs to be strictly controlled. Trace amounts of B significantly increase hardenability and promote bainite formation, which is not conducive to obtaining low hardness and quasi-polygonal ferrite.

[0016] Al: 0.020-0.040%. As a deoxidizing element, Al combines with N to form AlN, refining the grain size.

[0017] Ca: 0.001-0.004%. Ca treatment modifies non-metallic inclusions into spherical calcium aluminates, improving the morphology of the inclusions.

[0018] N: ≤0.002%. The ultra-low N content prevents premature precipitation of fine AlN or Nb (C,N) particles, maintaining sufficient microalloy solid solution.

[0019] The L450M pipeline steel of this invention, characterized by its resistance to large deformation and low hardness, has a dual-phase microstructure consisting of quasi-polygonal ferrite and degenerate pearlite. The quasi-polygonal ferrite, acting as a soft matrix with fine equiaxed grains, provides excellent plastic deformation capacity and accounts for 65%-85% of the material. The degenerate pearlite, acting as a strength-contributing phase, is dispersed along the ferrite grain boundaries or within the grains, providing necessary strength support and accounting for 15%-35%. This microstructure ensures both high uniform elongation (≥11%) and low yield strength ratio (≤0.77), while also achieving low hardness (≤210HV10, average <200HV10), which is key to achieving the synergistic goal of "high strength-low hardness-high deformation resistance" in this invention.

[0020] Furthermore, the grain size of the pipeline steel is ≥9, ensuring good comprehensive mechanical properties.

[0021] The pipeline steel has a yield strength ≥460MPa, tensile strength ≥600MPa, uniform elongation ≥11%, yield strength ratio ≤0.80 (preferably ≤0.77), and hardness ≤210HV10 (preferably average value <200HV10). Furthermore, the pipeline steel possesses good low-temperature toughness and processing performance, meeting the requirements for different thickness specifications (example thickness range is 9-21mm).

[0022] The production method of the L450M pipeline steel with high deformation resistance and low hardness is characterized by the following main process steps: Step (1): Smelting and continuous casting Steelmaking employs refining processes including KR+BOF+LF+RH. During the RH treatment, calcium treatment modifies inclusions, controlling the level of various non-metallic inclusions to ≤0.5. After smelting, slabs are continuously cast. The superheating temperature is 18-28℃, the casting speed is 0.9-1.3m / s, and dynamic light reduction is implemented. Vacuum degassing and calcium treatment remove gases, promoting the spheroidization and modification of non-metallic inclusions, ensuring the extreme purity of the steel. The low superheating combined with the light reduction process inhibits columnar crystal growth, eliminates central segregation and porosity, and controls the Mannesmann rating of the slab to ≤1.

[0023] Step (2): Heating and rolling (2-1) Heating: The soaking temperature is 1180-1220℃, and the holding time is 3.5-4.5h. This temperature range allows Nb and Ti carbonitrides to completely dissolve into the austenite, preparing for subsequent precipitation pinning. At the same time, a temperature ≤1220℃ can prevent abnormal growth of austenite grains. After heating, a single, uniform austenite structure is formed.

[0024] (2-2) Rolling: The entire rolling process is divided into two stages: roughing and finishing. The initial rolling temperature for roughing is 1050-1100℃, with a cumulative reduction of ≥60%, and the final rolling temperature is 780-830℃ (in the non-recrystallization region of austenite). In terms of reduction distribution, the single-pass reduction rate for the first two passes is controlled at 15%-20%, and for the subsequent passes at 10%-15%. High reduction rolling causes severe deformation of austenite grains, forming high-density dislocation cells; Nb (C,N) dynamically precipitates and pins grain boundaries, resulting in ultra-fine deformed austenite without recrystallization coarsening, providing a large number of nucleation sites for ferrite phase transformation.

[0025] Step (3): Two-stage controlled cooling After rolling, the steel plate undergoes two-stage cooling in an accelerated cooling system: Initial slow cooling: The cooling rate is 5-15℃ / s. During this stage, a large amount of deformed austenite is converted into quasi-polygonal ferrite (soft phase matrix), with fine and equiaxed grains, providing the material with good plastic deformation capacity (including uniform elongation and low yield strength ratio). Slow cooling ends when the steel plate cools to 650-750℃, and the subsequent rapid cooling begins.

[0026] Rear-stage rapid cooling: The cooling rate is 15-30℃ / s. In this stage, the increased cooling rate inhibits further coarsening of ferrite, allowing the remaining carbon-rich austenite to undergo a rapid phase transformation. Carbon diffusion is restricted, preventing the formation of lamellar pearlite, ultimately resulting in degenerate pearlite (a strength-contributing phase).

[0027] After cooling, the product is conveyed by roller conveyor to the cooling bed for air cooling, with a final cooling temperature of 350-450℃.

[0028] Compared with the prior art, the present invention has the following beneficial effects: (1) The L450M pipeline steel of the present invention with high deformation resistance and low hardness has a grain size ≥ 9, yield strength ≥ 460 MPa, tensile strength ≥ 600 MPa, uniform elongation ≥ 11%, and yield strength ratio ≤ 0.77. It has high strength, good resistance to large deformation and good comprehensive mechanical properties.

[0029] (2) The L450M pipeline steel of the present invention with high deformation resistance and low hardness has a hardness ≤210HV10 and an average hardness <200HV10, which greatly improves the processing performance of high strength and high deformation resistance pipeline steel, effectively reduces the forming difficulty and equipment wear in the pipe making process, and reduces the pipe making cost.

[0030] (3) This invention achieves a synergistic match of “high strength, low hardness and high resistance to deformation”, and solves the problems of high hardness, difficult pipe forming and poor matching of deformation resistance and hardness in the prior art. At the same time, it does not add precious metal alloys such as Ni, Mo and Cu, and the alloy cost is low.

[0031] (4) This invention obtains a two-phase microstructure of quasi-polygonal ferrite + degenerate pearlite through precise matching of LF+RH refining-continuous casting-controlled rolling and controlled cooling processes, which not only ensures a high uniform elongation (≥11%) and a low yield strength ratio, but also achieves low hardness (≤210HV10). The production method has a clear process, well-defined parameters, and good repeatability, making it suitable for industrial production.

[0032] (5) The pipeline steel of the present invention is particularly suitable for harsh service scenarios such as areas with frequent geological activity, permafrost zones, and earthquake-prone areas where pipeline displacement is likely to occur. It can meet the comprehensive performance requirements of oil and gas long-distance pipelines for materials and solve the safety and economic problems caused by pipeline deformation failure in these environments. Attached Figure Description

[0033] Figure 1 This is a metallographic micrograph of Embodiment 1 of the present invention.

[0034] Figure 2 This is a metallographic micrograph of Embodiment 2 of the present invention.

[0035] Figure 3 This is a metallographic micrograph of Embodiment 3 of the present invention.

[0036] Figure 4 This is a metallographic micrograph of a comparative example of the present invention. Detailed Implementation

[0037] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0038] Example 1 (steel plate thickness 14.3mm) The L450M pipeline steel with high deformation resistance and low hardness in this embodiment has the following chemical composition by mass percentage: C: 0.05%, Si: 0.25%, Mn: 1.50%, P: 0.0018%, S: 0.0015%, Cr: 0.06%, Ni: 0.04%, Cu: 0.10%, Mo: 0.05%, Nb: 0.045%, V: 0.03%, Ti: 0.014%, B: 0.0003%, Al: 0.030%, Ca: 0.002%, N: 0.0015%, with the balance being Fe and unavoidable impurities.

[0039] The production method mainly includes the following technological steps: (1) Smelting and continuous casting Steelmaking employs a process including KR+BOF+LF+RH. Steelmaking raw materials undergo KR pretreatment for deep desulfurization; after BOF smelting, the steel is further desulfurized and alloy composition adjusted in the LF refining furnace; the RH treatment process modifies inclusions through calcium treatment, controlling the level of various non-metallic inclusions to ≤0.5.

[0040] After smelting, the slabs are continuously cast. The superheat during continuous casting is controlled within the range of 18-28℃. Protective casting is used throughout the process, and the casting speed is controlled between 0.9-1.3 m / s. Dynamic light pressure is used to avoid center segregation. The low-magnification Mannesmann rating of the slabs is ≤1.

[0041] (2) Heating and rolling (2-1) Heating: The temperature of the soaking zone is controlled at 1180-1220℃ (the actual temperature is about 1200℃), and the total heating time is 3.5-4.5h. This ensures that the carbonitrides of microalloying elements such as Nb and Ti are completely dissolved, while preventing abnormal growth of austenite grains. After heating, a single uniform austenite structure is presented.

[0042] (2-2) Rolling: The initial rolling temperature is approximately 1050-1100℃. After multiple passes in the roughing stage, finishing rolling is carried out. The final rolling temperature is controlled at 800-810℃, and the cumulative reduction in finishing rolling is ≥60%. In terms of reduction distribution, the single-pass reduction rate of the first two passes is controlled at 15%-20%, and the reduction rate of the subsequent passes is controlled at 10%-15% to ensure sufficient deformation of the core structure.

[0043] (3) Two-stage controlled cooling After rolling, the steel plate is cooled in two stages using water-cooling equipment: The front section adopts a slow cooling method with a cooling rate of 5-15℃ / s (actually measured at about 10℃ / s), which promotes the formation of a large amount of quasi-polygonal ferrite from deformed austenite. The latter part adopts a rapid cooling method with a cooling rate of 15-30℃ / s (actually measured at about 20℃ / s), which inhibits the continued coarsening of ferrite and allows the remaining carbon-rich austenite to undergo rapid phase transformation to generate degenerate pearlite.

[0044] The final cooling temperature is controlled at 380-420℃ (actually measured at about 400℃). After water cooling, the water is conveyed to the cooling bed by roller conveyor for air cooling.

[0045] Performance testing Metallurgical microscopy revealed a microstructure of quasi-polygonal ferrite + degenerate pearlite with a grain size of 9.5. Tensile properties were tested according to API Spec 5L, showing a yield strength of 470 MPa, a tensile strength of 625 MPa, a uniform elongation of 11.5%, and a yield-to-tensile ratio of 0.75. Vickers hardness testing (9 points, HV10) according to GB / T 4340.1 showed hardness values ​​ranging from 187 to 203 HV10, with an average hardness of 196 HV10. All results were superior to the target specifications.

[0046] Example 2 (steel plate thickness 9mm) The chemical composition of this embodiment is basically the same as that of Example 1, and the specific component mass percentages are as follows: C: 0.048%, Si: 0.28%, Mn: 1.55%, P: 0.0015%, S: 0.0012%, Cr: 0.05%, Ni: 0.03%, Cu: 0.08%, Mo: 0.04%, Nb: 0.050%, V: 0.02%, Ti: 0.016%, B: 0.0002%, Al: 0.035%, Ca: 0.0025%, N: 0.0012%, with the balance being Fe and unavoidable impurities.

[0047] The preceding processes are the same as in Example 1, but the key process parameters are controlled as follows: The final rolling temperature is controlled at 790-800℃, and the cumulative reduction in finishing rolling is ≥60%. Cooling process: the initial slow cooling rate is 5-15℃ / s (actually measured at about 8℃ / s), the subsequent rapid cooling rate is 15-30℃ / s (actually measured at about 22℃ / s), the final cooling temperature is controlled at 420-450℃ (actually measured at about 435℃), followed by air cooling on a cooling bed.

[0048] Performance testing: The metallographic structure consists of quasi-polygonal ferrite and degenerate pearlite, with a grain size of grade 9. The yield strength is 490 MPa, the tensile strength is 642 MPa, the uniform elongation is 10.9%, and the yield-to-tensile ratio is 0.76. Hardness testing (9 points, HV10) shows a distribution between 192-206 HV10, with an average hardness of 198 HV10.

[0049] Example 3 (steel plate thickness 21mm) The chemical composition of this embodiment is basically the same as that of Example 1, and the specific component mass percentages are as follows: C: 0.055%, Si: 0.30%, Mn: 1.45%, P: 0.0012%, S: 0.0010%, Cr: 0.08%, Ni: 0.05%, Cu: 0.12%, Mo: 0.06%, Nb: 0.040%, V: 0.04%, Ti: 0.012%, B: 0.0004%, Al: 0.025%, Ca: 0.0030%, N: 0.0016%, with the balance being Fe and unavoidable impurities.

[0050] The preceding processes are the same as in Example 1, but the key process parameters are controlled as follows: The final rolling temperature is controlled at 810-820℃, and the cumulative reduction in finishing rolling is ≥60%. Cooling process: the initial slow cooling rate is 5-15℃ / s (actual measurement is about 12℃ / s), the subsequent rapid cooling rate is 15-30℃ / s (actual measurement is about 25℃ / s), the final cooling temperature is controlled at 350-380℃ (actual measurement is about 365℃), followed by air cooling on a cooling bed.

[0051] Performance testing: The metallographic structure consists of quasi-polygonal ferrite and degenerate pearlite, with a grain size of 9.5. The yield strength is 465 MPa, the tensile strength is 615 MPa, the uniform elongation is 11.6%, and the yield-to-tensile ratio is 0.76. Hardness testing (9 points, HV10) shows a distribution between 185-195 HV10, with an average hardness of 190 HV10.

[0052] Comparative example (steel plate thickness 14.3mm) To verify the effectiveness of the two-stage cooling process of this invention, a comparative example was set up. The chemical composition and preceding processes (smelting, continuous casting, heating, and rolling) of the comparative example were completely consistent with those of Example 1. After rolling, a single-stage rapid cooling method was adopted, with the cooling rate controlled at 15-30℃ / s and the final cooling temperature controlled at 380-420℃ (actually measured at approximately 393℃), followed by air cooling on a cooling bed.

[0053] Performance testing: Bainite and M / A island hard phases are present in the metallographic structure. The yield strength is 510 MPa, the tensile strength is 605 MPa, the uniform elongation is only 8.5%, and the yield strength ratio is 0.84. Hardness tests (9 points, HV10) show a distribution between 196-218 HV10, with an average hardness of 207 HV10. Although the tensile strength still meets the requirements, the uniform elongation drops significantly, and the average hardness is higher than 210 HV10, failing to meet the comprehensive requirements of low hardness and resistance to large deformations.

[0054] Table 1 Tensile properties of pipeline steel in the examples and comparative examples

[0055] Table 2. Pipeline steel hardness values ​​(9 points, HV10) for the examples and comparative examples

[0056] Table 3 Charpy impact properties of pipeline steel in the examples and comparative examples (-40℃)

[0057] Table 4. Drop hammer performance of pipeline steel in the examples and comparative examples (-20℃)

[0058] Results Analysis The comparison data between the examples and the comparative examples clearly show that the two-stage controlled rolling and cooling process of the present invention (Examples 1-3) can stably control the material hardness below 210 HV10 (average value < 200 HV10), while maintaining a high uniform elongation of ≥11%. In contrast, the comparative examples, using conventional single-stage rapid cooling, resulted in the formation of bainite and M / A island hard phases in the microstructure, causing the average hardness to jump to 207 HV10, and the uniform elongation to drop significantly to 8.5%, failing to meet the comprehensive requirements of high deformation resistance and low hardness.

[0059] The above experimental data fully demonstrate the effectiveness and superiority of the technical solution of this invention. Through the precise matching of LF+RH refining-continuous casting-controlled rolling and cooling processes, the technical solution of this invention achieves a synergistic effect of "high strength, low hardness, and high deformation resistance," and has good prospects for industrial application.

[0060] In addition to the above embodiments, the present invention can also adjust parameters such as billet thickness and continuous casting process according to the production requirements of converters and electric furnaces of different tonnages. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A low-hardness L450M pipeline steel with resistance to large deformation, characterized in that, The chemical composition by mass percentage is as follows: C: 0.04-0.06%, Si: 0.15-0.35%, Mn: 1.35-1.65%, P≤0.002%, S≤0.002%, Cr:≤0.1%, Ni:≤0.1%, Cu:≤0.20%, Mo:≤0.1%, Nb: 0.035-0.055%, V:≤0.1%, Ti: 0.008-0.020%, B:≤0.0005%, Al: 0.020-0.040%, Ca: 0.001-0.004%, N≤0.002%, with the balance being Fe and unavoidable impurities.

2. The L450M pipeline steel with resistance to large deformation and low hardness according to claim 1, characterized in that, The microstructure of the pipeline steel is a two-phase structure of quasi-polygonal ferrite and degenerate pearlite, with quasi-polygonal ferrite accounting for 65%-85% and degenerate pearlite accounting for 15%-35%, and grain size ≥ 9.

3. The L450M pipeline steel with resistance to large deformation and low hardness according to claim 1 or 2, characterized in that, The pipeline steel has a yield strength ≥460MPa, tensile strength ≥600MPa, uniform elongation ≥11%, yield strength ratio ≤0.80, and hardness ≤210HV10.

4. The L450M pipeline steel with resistance to large deformation and low hardness according to claim 3, characterized in that, The average hardness of the pipeline steel is less than 200HV10.

5. A method for producing L450M pipeline steel with resistance to large deformation and low hardness, characterized in that, The main process steps include the following: (1) Smelting and continuous casting: Smelting is carried out according to the chemical composition described in claim 1, and LF+RH refining and calcium treatment are adopted. After smelting, the slab is continuously cast. The superheat of continuous casting is 18-28℃, the casting speed is 0.9-1.3m / s, and dynamic light pressure is implemented. (2) Heating and rolling: The temperature of the soaking section is 1180-1220℃ and the holding time is 3.5-4.5h; the whole rolling process is divided into two stages: rough rolling and finish rolling. The initial rolling temperature of rough rolling is 1050-1100℃, the cumulative reduction is ≥60%, and the final rolling temperature is 780-830℃. (3) Two-stage controlled cooling: After rolling, two-stage cooling is carried out in the accelerated cooling system. The first stage adopts slow cooling with a cooling rate of 5-15℃ / s; the second stage adopts fast cooling with a cooling rate of 15-30℃ / s and a final cooling temperature of 350-450℃. After water cooling, air cooling is carried out.

6. The method for producing L450M pipeline steel with resistance to large deformation and low hardness according to claim 5, characterized in that, In the rolling process of step (2), the single-pass reduction rate of the first two passes is controlled at 15%-20%, and the reduction rate of the subsequent passes is controlled at 10%-15%.

7. The production method of L450M pipeline steel with high deformation resistance and low hardness according to claim 5, characterized in that: In step (1), the low-magnification Mannesmann rating of the billet is controlled to be ≤1, and the level of various non-metallic inclusions is controlled to be ≤0.

5.

8. The method for producing L450M pipeline steel with high deformation resistance and low hardness according to claim 5, characterized in that, In step (3), the slow cooling ends when the steel plate is cooled to 650-750℃, and the subsequent rapid cooling begins.

Images