An ultra-thin gauge economic x65m pipeline steel plate with excellent low-temperature toughness and a preparation method thereof

Abstract

This invention discloses an ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness and its preparation method, relating to the field of low-carbon microalloy steel production technology. The steel composition includes: C: 0.04%–0.06%, Si: 0.15%–0.25%, Mn: 1.45%–1.60%, P≤0.015%, S≤0.01%, Nb: 0.03%–0.05%, Ti: 0.015%–0.025%, Al: 0.015%–0.04%, N: 0.003%–0.006%, with the balance being Fe and unavoidable impurities. This invention reduces alloy costs through simple composition design and achieves the preparation of pipeline steel plates with a thickness of 6–8.5 mm, exhibiting low cost, high strength, and excellent low-temperature toughness by controlling the steelmaking, continuous casting, and controlled rolling and cooling processes, thus ensuring the safe operation of gas pipelines.

Description

Technical Field

[0001] This invention relates to the field of low-carbon microalloyed steel production technology, and more specifically, to an ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness and its preparation method. Background Technology

[0002] In recent years, the construction of energy transmission pipelines has developed rapidly. To improve transmission efficiency and operational safety, the requirements for the strength and toughness of the selected steel raw materials are becoming increasingly stringent. Thin-gauge pipeline steel has seen rapid development and application in recent years due to its economic advantages in weight reduction, transportation, and production. Wall thicknesses have expanded to extremely thin specifications of 6-8.5 mm. When pipelines are located in extremely cold regions, from the perspective of operational stability and safety, the thinner the wall, the more prone it is to fracture under external forces. How to prevent brittle fracture in low-temperature environments, thereby improving service life and safety, has become a crucial issue in the research, development, production, and application of ultra-thin-gauge pipeline steel. Typically, the compression ratio for rolling thin-gauge pipeline steel is above 20. This high compression ratio leads to a greater concentration of defects such as center segregation in the core of the thin-gauge steel plate (ferrite and pearlite are distributed in bands along the rolling direction at different locations on the plate thickness section, with banding levels greater than 3.5 and even reaching 4). During external deformation, stress concentration easily occurs here, inducing cracks on the lower surface where tensile stress is greatest, thus worsening drop hammer performance. Furthermore, due to the rapid temperature drop of thin-gauge steel plates, it is difficult to achieve sufficient rolling control, further complicating the control of low-temperature DWTT performance. Currently, to ensure the low-temperature toughness of thin-gauge pipeline steel, a relatively thick intermediate billet is usually maintained for heating, and a two-stage or even three-stage controlled rolling process is adopted, along with increased deformation per rolling pass to control the banded structure of the pipeline steel. This results in the final rolling temperature of the steel plate potentially being controlled below 820℃, or even lower, significantly increasing the rolling difficulty. Firstly, mill load, steel plate temperature uniformity, and plate shape control all face severe challenges, especially since the thickness of the steel plates is mainly above 9mm. Therefore, the key to developing ultra-thin, economical X65M pipeline steel lies in solving the problems existing in the production of X65M pipeline steel with a thickness of 6~8.5mm and a pipe diameter of Φ914mm, and ensuring its excellent low-temperature toughness.

[0003] Patent application number 201010243241.0 discloses an X65 pipeline steel and its production method, with the following chemical composition: C: 0.055~0.090wt%, Si: 0.15~0.35wt%, Mn: 1.50%~1.65wt%, P≤0.020wt%, S≤0.005wt%, Nb: 0.040%~0.055wt%, V: 0.040%~0.070wt%, Ti: 0.010%~0.025wt%, N≤0.008wt%, Als: 0.005%~0.060wt%. This steel has a high V content, and the process uses natural air cooling, which is inefficient and cannot fully utilize the water-based alloying to reduce costs and improve strength and toughness. Furthermore, the specific strength, toughness, and manufacturing thickness intervals of the steel are not clearly defined. Patent application number 201110179945.0 discloses an X65 pipeline steel with excellent low-temperature toughness and its manufacturing method. The weight percentage composition of this patent is: C: 0.020%–0.055%, Si: 0.10%–0.25%, Mn: 1.50%–1.70%, Nb: 0.060%–0.080%, Cr: 0.20%–0.35%, V: 0.020%–0.040%, Ti: 0.010%–0.020%, Als: 0.015%–0.040%, P≤0.018%, S≤0.005%, N≤0.006%. This steel contains V, resulting in high alloy costs. The product is a coil, not a flat plate, and the thickness spacing is not clearly defined. Patent application number KR20020027013 discloses a steel product for stress relief heat treatment guaranteed API-X65 grade linepipes and method for manufacturing the same. The patented composition by weight percentage is C: 0.07%–0.09%, Si: 0.2%–0.5%, Mn: 1.40%–1.60%, P≤0.025%, S≤0.005%, Nb: 0.035%–0.045%, V: 0.04%–0.05%, Mo: 0.03%–0.07%, Cr: 0.05%–0.15%, Ti: 0.005%–0.015%, N: 0.002%–0.007%, Als: 0.015%–0.050%. This steel contains V and Mo, resulting in high cost and insufficient low-temperature toughness in the actual product.Patent application number JP2005194607 discloses a high-strength steel sheet for linepipe superior in high-speed ductile fracture resistance and manufacturing method therefor. The weight percentage composition of this patent is: C: 0.02%–0.08%, Si: 0.01%–0.5%, Mn: 0.5%–1.8%, P≤0.01%, S≤0.002%, Nb: 0.005%–0.05%, Mo: 0.005%–0.5%, Ti: 0.005%–0.04%, N: 0.002%–0.007%, Al≤0.07%, Mg: 0.0005%–0.005%. This steel contains Mo and Mg, resulting in high cost, and the method for controlling banded microstructure is not clearly defined.

[0004] Although the steel disclosed in the above patent documents has achieved high strength and toughness, it is either limited to coiled plates, or has high production costs and unclear manufacturing thickness intervals, and lacks a manufacturing process for X65M economical ultra-thin pipeline steel with good low-temperature toughness. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned defects in the prior art and provide an ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness and its preparation method. This invention enables the preparation of pipeline steel plates with high strength and excellent low-temperature toughness. The steel plate can be used to manufacture steel pipes with a diameter of Φ914mm and below, which can ensure the safe operation of gas pipelines.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] An ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness comprises the following components by weight percentage: C: 0.04%–0.06%, Si: 0.15%–0.25%, Mn: 1.45%–1.60%, P≤0.015%, S≤0.01%, Nb: 0.03%–0.05%, Ti: 0.015%–0.025%, Al: 0.015%–0.04%, N: 0.003%–0.006%. The weight percentage ratio of C, Si, Ti, Al, N, Nb, and Mn in the X65M pipeline steel plate conforms to: 3.40≤0.7(Ti / N)+0.5e （0.3Nb-Al） +0.75(2.4C-1.3Si+0.2Mn)≤4.00, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%.

[0008] This invention also discloses a method for preparing ultra-thin, economical X65M pipeline steel plates with excellent low-temperature toughness as described above, including: KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling and air cooling to room temperature;

[0009] S1. The smelting raw materials are sequentially subjected to KR hot metal pretreatment, converter smelting, LF refining and RH vacuum degassing to obtain molten steel;

[0010] S2. In the continuous casting process, the superheat is 8-15℃, the casting speed is 0.9-1.3m / min; the electromagnetic stirring current intensity in the secondary cooling zone is controlled at 150-200A, the secondary cooling water volume is 1.8-2.2L / kg, and the columnar crystal ratio in the continuous casting billet is controlled at ≤15%; in the horizontal fan-shaped section, i.e. the end of solidification, a light pressure is applied, the reduction of the continuous casting billet is 5-10mm, and after the billet is cast, it is stacked and slowly cooled, the stacking temperature is ≥650℃, and the slow cooling time is ≥48h, to obtain the cooled billet;

[0011] S3. In the reheating of the billet, the cooled billet is passed sequentially through a preheating section, a heating section, and a soaking section before exiting the furnace to obtain a heated billet; wherein, the temperature range of the preheating section is 850-1100℃, the temperature range of the heating section is 1270-1295℃, the temperature range of the soaking section is 1250-1270℃, and the total furnace time of the heating section and the soaking section is 3.5-4.5h;

[0012] S4. During the rolling and cooling processes, before rolling begins, the heated billet is descaled using high-pressure water after exiting the furnace. The rolling roll cycle is selected as 1500-2500 tons after roll change. Simultaneously, the cooling water in the front and rear roller tables of the rolling mill is shut off, and the flow rate of the rolling roll cooling water is controlled at 25-50 m³ / h. 3 The steel is rolled into a plate in two stages, in the recrystallization zone and the non-recrystallization zone respectively. The initial rolling temperature of the first stage is 1200-1220℃, the final rolling temperature of the first stage is 1030-1060℃, the number of rolling passes in the first stage is no more than 5, the reduction rate of each pass in the first and second passes is ≥35%, the thickness of the intermediate billet is 30-55.25mm, and the thickness of the intermediate billet is 5-6.5 times the thickness of the finished product. The initial rolling temperature of the second stage is 910–930℃, and the final rolling temperature of the second stage is 730–750℃. The number of rolling passes in the second stage shall not exceed 4, and the reduction rate of each pass in the first and second passes shall be ≥15%. After rolling, the steel is rapidly ejected at a speed of 6–7.5 m / s, and then cooled by laminar flow cooling at an initial cooling temperature of 650–680℃ and a final cooling temperature of 450–480℃, with a cooling rate of 5–10℃ / s.

[0013] Implementing the embodiments of the present invention will have the following beneficial effects:

[0014] 1. After deep desulfurization pretreatment of KR hot metal, the slag is removed cleanly. The converter adopts the double slag method to remove P, so that the P and S content of the billet is low. The argon blowing time is controlled and the RH vacuum degassing time is maintained, thereby overcoming the defects such as central segregation, inclusions and excessive H and O content in the billet caused by high Mn content. This is beneficial to improving the plasticity and toughness of pipeline steel. Reducing superheat, lowering continuous casting speed, and applying light pressure can improve macroscopic segregation in continuously cast billets, reduce the spacing of secondary dendrite arms in the solidification structure of the billet, help reduce billet segregation, reduce internal structural defects, refine austenite grains, and reduce the width of ferrite bands in the banded structure after rolling. Optimizing the electromagnetic stirring current intensity and the secondary cooling water ratio in the secondary cooling zone can reduce the average carbon segregation index, suppress segregation, and reduce the proportion of columnar crystals, keeping it below 15%. By limiting the intensity of secondary cooling, the tendency of central segregation and crack deterioration in the billet can be suppressed. Billet stacking helps reduce central segregation in the billet and effectively reduces the internal H content of the billet.

[0015] 2. Billet reheating: The temperature and time of the billet in the preheating, heating and soaking sections are limited to ensure that the carbides and nitrides of Nb and Ti are quickly and fully dissolved in the matrix and fully diffused. At the same time, it promotes the diffusion of alloying elements Mn and Cr, reduces the impact of their compositional segregation on the microstructure and properties, and further ensures the temperature of the billet at different stages of rolling.

[0016] 3. The composition of this invention is reasonable and the amount of alloy added is low. It uses low carbon and medium manganese, and adds a small amount of micro-alloying elements Nb and Ti, which greatly reduces the alloy cost and the resistance to high-temperature deformation in the roughing and finishing stages. This is conducive to increasing the reduction in each pass and ensuring the comprehensive performance of the super steel plate.

[0017] 4. Control the descaling mill pressure and time to avoid excessive temperature drop in the rolled steel plate; control the production roll cycle to control the roll crown and ensure the shape of the rolled steel plate; simultaneously shut off the cooling water in the front and rear roller tables of the mill to reduce the flow rate of the cooling water, thereby reducing the contact between the steel plate and the cooling medium and preventing excessive temperature drop in the rolled steel plate. A two-stage controlled rolling process is adopted to control the reduction in the roughing and finishing passes, optimize the thickness of the intermediate slab, extend the waiting time of the intermediate slab, ensure the core rolling penetration, and suppress the formation of wide banded structures in the core; through two-stage rolling, the grain structure at different stages is refined and pre-deformed to a certain extent to reduce the grain size after phase transformation; the final rolling temperature is controlled to ensure the proportion of large-angle grain boundaries and improve the strength and toughness of the steel plate. By adopting a relatively low initial cooling temperature, the proportion of polymorphic ferrite in the steel plate after final cooling is ensured to be above 30%, of which the proportion of large-angle grain boundaries with a crystal orientation difference of more than 15° in the metallographic structure is ≥60%. The fine polygonal ferrite structure with large-angle grain boundaries can effectively reduce the average effective grain size of the steel. At the same time, the slow cooling rate avoids the formation of M / A phase in the structure, ensuring that the proportion of M / A phase is within 3%, while inhibiting the diffusion and segregation of carbon elements and avoiding the formation of banded structure.

[0018] 5. This invention reduces alloy costs through simple composition design and achieves an economical X65M pipeline steel plate with excellent low-temperature toughness for gas pipelines and its preparation method by controlling steelmaking, continuous casting, and controlled rolling and cooling processes. This steel plate exhibits low cost, high strength, and high toughness. The microstructure of the steel plate is a multiphase structure mainly composed of pearlite and polygonal ferrite, with polygonal ferrite accounting for more than 30% and the M / A phase ratio less than 3%. This steel plate possesses low cost, high strength, and high toughness, ensuring the safe operation of oil pipelines. Specific properties include: transverse tensile yield strength between 470 and 510 MPa, tensile strength between 570 and 630 MPa (narrow strength range), and elongation ≥25%. The transverse Charpy impact energy at -30℃ is ≥180 J, the DWTT transverse shear area (SA) at -30℃ is ≥90%, and the banded structure is grade 3 or below. Detailed Implementation

[0019] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0020] This invention discloses an ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness, comprising the following components by weight percentage: C: 0.04%–0.06%, Si: 0.15%–0.25%, Mn: 1.45%–1.60%, P≤0.015%, S≤0.01%, Nb: 0.03%–0.05%, Ti: 0.015%–0.025%, Al: 0.015%–0.04%, N: 0.003%–0.006%. The weight percentage ratio of C, Si, Ti, Al, N, Nb, and Mn in the X65M pipeline steel plate conforms to: 3.40≤0.7(Ti / N)+0.5e （0.3Nb-Al） +0.75(2.4C-1.3Si+0.2Mn)≤4.00, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%.

[0021] Specifically, the roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0022] C: The most economical and basic strengthening element in steel, it has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content has a negative impact on the plasticity, toughness, and weldability of steel. Therefore, this invention sets the C content range to 0.04% to 0.06%.

[0023] Mn: Mn strengthens steel through solid solution treatment, compensating for the strength loss caused by the reduction in carbon content. Furthermore, it lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to finer low-temperature transformation products, thus improving toughness. However, increasing the Mn content exacerbates center segregation in continuously cast billets, hindering the improvement of low-temperature toughness and compromising the uniformity of the cross-sectional microstructure. Therefore, the Mn content range in this invention is designed to be 1.45%–1.60%.

[0024] Si (Si) plays a role in deoxidation in steelmaking and improving the strength of the matrix. However, excessive Si will reduce the toughness of the heat-affected zone in the base metal. Increasing the Si content can purify ferrite and reduce the content of pearlite, which is beneficial for reducing the Bauschinger effect in the matrix material. Therefore, the Si content is set at 0.15% to 0.25% in this invention.

[0025] Nitrogen (Nb) is a commonly used element in modern microalloyed pipeline steel, exhibiting excellent grain refinement and precipitation strengthening effects; it also delays austenite recrystallization. However, excessive Nb increases production costs and complicates continuous casting process control. This invention selects an Nb content range of 0.03% to 0.05%, combined with a suitable TMCP process, to obtain a uniform composite phase dominated by pearlite, polygonal ferrite, and acicular ferrite, resulting in good toughness.

[0026] N: In addition to forming fine TiN particles to refine the austenite grains, the N element in steel needs to be kept at a low content level. The N content range selected in this invention is 0.003% to 0.006%.

[0027] Ti is a strong solid nitrogen element, existing in the form of TiN in continuously cast billets. Fine TiN particles can effectively inhibit austenite grain growth during reheating of the continuously cast billet and help improve the solid solubility of Nb in austenite, thus improving the impact toughness of the weld heat-affected zone. When the Ti content exceeds a certain value, the TiN particles coarsen, increasing the stress concentration level at the particle interface and the matrix. Therefore, this invention selects a Ti content range of 0.015% to 0.025%.

[0028] Al: Commonly used as a deoxidizer in steel, it can also refine the microstructure if it forms AlN. When the Al content exceeds 0.04%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. Therefore, the lower limit for Al content is set at 0.015%.

[0029] P and S are unavoidable impurity elements in steel, and their levels should be as low as possible. However, due to considerations of smelting costs and processes, they cannot be kept infinitely low. Therefore, this invention controls P ≤ 0.015% and S ≤ 0.01%.

[0030] Furthermore, this invention also adjusts the relationship: 3.40 ≤ 0.7(Ti / N) + 0.5e （0.3Nb-Al） +0.75(2.4C-1.3Si+0.2Mn)≤4.00, by precisely controlling the ratio of elements, both high strength and excellent low-temperature toughness are achieved.

[0031] In one specific embodiment, the yield strength of the pipeline steel plate in the transverse tensile is 470-510 MPa, the tensile strength is 570-630 MPa, the elongation is ≥25%, the transverse Charpy impact energy at -30℃ is ≥180 J, and the transverse shear area SA at -30℃ is ≥90%.

[0032] In one specific embodiment, the thickness of the pipeline steel plate is 6~8.5mm.

[0033] In one specific embodiment, in the metallographic structure of the pipeline steel plate, the volume fraction of polygonal ferrite is above 30%, the volume fraction of M / A phase is controlled within 3%, the remainder is pearlite, the banded structure is ≤ grade 3, and the proportion of large-angle grain boundaries with a crystal orientation difference of more than 15° in the metallographic structure is ≥ 60%.

[0034] This invention also discloses a method for preparing ultra-thin, economical X65M pipeline steel plates with excellent low-temperature toughness as described in any embodiment of this invention, comprising: KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling, and air cooling to room temperature; specifically including:

[0035] S1. The smelting raw materials are sequentially subjected to KR molten iron pretreatment, converter smelting, LF refining and RH vacuum degassing to obtain molten steel.

[0036] In one specific embodiment, step S1 includes: pretreating the smelting raw materials with KR molten iron to control the S content ≤0.01%, and then feeding them into the converter after slag removal; using the double slag method to remove P during converter smelting to control the P content ≤0.015%, controlling the C content to be 0.04%~0.06% at the end of converter smelting, and blowing argon gas for 20~25 minutes when tapping steel from the converter; subsequently performing LF refining and RH vacuum degassing in sequence, with RH vacuum degassing maintained for ≥30 minutes.

[0037] S2. In continuous casting, the superheat is 8–15℃, and the billet pulling speed is 0.9–1.3 m / min. The electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage is controlled at 150–200 A, and the secondary cooling water volume is 1.8–2.2 L / kg. This reduces the average carbon segregation index and suppresses segregation. By limiting the intensity of secondary cooling, the tendency of central segregation and crack deterioration in the billet is suppressed, and the proportion of columnar crystals is reduced. The proportion of columnar crystals in the continuous casting billet is controlled to be ≤15%. In the horizontal fan-shaped section, i.e., at the end of solidification, a light pressure is applied, and the reduction of the continuous casting billet is 5–10 mm. After the billet is produced, it is stacked and slowly cooled. The stacking temperature is ≥650℃, and the slow cooling time is ≥48 h. This promotes the diffusion of Mn and C elements and reduces their influence on the microstructure and properties due to component segregation, resulting in a cooled billet.

[0038] In one specific embodiment, in step S2, the thickness of the continuous casting billet is 135~170mm, and the continuous casting billet with a thickness of 135~170mm is used to produce on a medium and heavy plate reciprocating rolling mill, with water as the cooling medium.

[0039] S3. In the reheating of the billet, the cooled billet is passed through a preheating section, a heating section, and a soaking section before being taken out of the furnace to obtain a heated billet. The temperature range of the preheating section is 850-1100℃, which promotes the rapid and complete solidification of Nb and Ti carbides and nitrides into the matrix and allows for sufficient diffusion. The temperature range of the heating section is 1270-1295℃, and the temperature range of the soaking section is 1250-1270℃. The total furnace time in the heating and soaking sections is 3.5-4.5 hours, which further promotes the diffusion of alloying elements Mn, C, and H and reduces their influence on the microstructure and properties due to compositional segregation.

[0040] S4. During rolling and cooling, before the start of rolling, the heated billet is descaled using high-pressure water after exiting the furnace. The rolling roll cycle is selected to be 1500-2500 tons after roll changing. The production roll cycle is controlled to control the roll crown and ensure the shape of the rolled steel plate. At the same time, the cooling water in the front and rear roller tables of the rolling mill is shut off, and the flow rate of the rolling roll cooling water is controlled at 25-50 m³ / h. 3 / h, reducing the contact between the steel plate and the cooling medium to avoid excessively rapid temperature drop in the rolled steel plate; the steel plate is rolled in two stages, in the recrystallization zone and the non-recrystallization zone respectively. The initial rolling temperature of the first stage (rough rolling) is 1200~1220℃, and the final rolling temperature of the first stage is 1030~1060℃. The number of rolling passes in the first stage is no more than 5, and the reduction rate of each pass in the first and second passes is ≥35%. The thickness of the intermediate billet obtained in the first stage is 30~55.25mm. The intermediate billet thickness is increased by 5 to 6.5 times to reduce its temperature drop and ensure sufficient heating time. The initial rolling temperature of the second stage (finish rolling) is 910–930℃, and the final rolling temperature is 730–750℃. The second stage has no more than four rolling passes. The reduction rate in each pass of the first and second stages is ≥15% to ensure core penetration and promote dynamic recrystallization of austenite grains. Furthermore, the reduction gradually increases, causing the rolled piece to flatten and elongate. Simultaneously, due to dynamic… The combined effect of static and dynamic recrystallization allows new grains to nucleate and grow at grain boundaries, forming fine new recrystallized grains and suppressing band width. Two-stage rolling refines the grain structure at different stages and introduces a degree of pre-deformation to reduce the grain size after phase transformation. Controlling the final rolling temperature ensures a high proportion of large-angle grain boundaries, improving the strength and toughness of the steel plate. Rapid post-rolling steel blasting at a speed of 6–7.5 m / s is followed by laminar flow cooling, with an initial cooling temperature of 650–680℃ and a final cooling temperature of 45℃. The cooling temperature ranges from 0 to 480℃, with a cooling rate of 5 to 10℃ / s. The relatively low initial cooling temperature ensures that the proportion of polymorphic ferrite in the steel plate is above 30% after final cooling. Among them, the proportion of large-angle grain boundaries with a crystal orientation difference of more than 15° in the microstructure is ≥60%. The fine polygonal ferrite microstructure with large-angle grain boundaries can effectively reduce the average effective grain size of the steel. At the same time, the slow cooling rate avoids the formation of M / A phase in the microstructure, ensuring that the M / A phase ratio is within 3%, while inhibiting the diffusion and segregation of carbon elements and avoiding the formation of banded structures.

[0041] In one specific embodiment, in step S4, the high-pressure water descaling time is 0.5 to 1 minute, and the high-pressure water descaling pressure is 10 to 15 MPa. The pressure and time of the descaling machine are controlled to avoid the temperature of the rolled steel plate dropping too quickly.

[0042] Specifically, this invention employs the above-mentioned composition and a process involving KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling, and air cooling to room temperature to produce pipeline steel plates with a thickness of 6-8.5 mm that possess low cost, high strength, and excellent low-temperature toughness. These steel plates can be used to manufacture steel pipes with a diameter of Φ914 mm and below, ensuring the safe operation of gas pipelines. The microstructure of the steel plate is a multiphase structure dominated by pearlite and polygonal ferrite, with the volume fraction of polygonal ferrite exceeding 30%, the volume fraction of the M / A phase controlled below 3%, and the remainder being pearlite. The banded structure is ≤3 grade, and the proportion of large-angle grain boundaries with a crystal orientation difference of 15° or more in the microstructure is ≥60%.

[0043] The following are specific embodiments.

[0044] Examples 1-6

[0045] The preparation method of the ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness in Examples 1-6 includes: KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling, and air cooling to room temperature; specifically including:

[0046] S1. After the raw materials for smelting are pretreated by KR hot metal for deep desulfurization, the slag is removed and the raw materials are then fed into the converter. During the converter smelting, the double slag method is used to remove phosphorus. The carbon content is controlled at the end of the converter smelting. Argon gas is blown when the steel is tapped from the converter. Then, LF refining and RH vacuum degassing are carried out in sequence to obtain molten steel.

[0047] S2. Molten steel is continuously cast. After the billet is produced, it is stacked and cooled slowly to obtain a cooled billet.

[0048] S3. In the reheating of the billet, the cooled billet is sent into the walking beam furnace and passes through the preheating section, heating section and soaking section in sequence before exiting the furnace to obtain the heated billet.

[0049] S4. During rolling and cooling, before the start of rolling, the heated billet is descaled using high-pressure water after exiting the furnace. The rolling roll cycle is selected as 1500-2500 tons after roll change. At the same time, the cooling water of the front and rear roller tables of the rolling mill is shut off, and the cooling water of the rolls is reduced to 25-50m. 3 / h; the steel is rolled into a plate in two stages, in the recrystallization zone and the non-recrystallization zone respectively; after rolling, the steel is quickly shot out and then cooled by laminar flow cooling and air-cooled to room temperature.

[0050] Table 1 shows the chemical composition of the steels in Examples 1-6; Table 2 shows the smelting and stacking process parameters of the steels in Examples 1-6; Table 3 shows the process parameters for reheating the cast billets and high-pressure water descaling of the continuously cast billets before rolling in Examples 1-6; Table 4 shows the rolling parameters of the steels in Examples 1-6; Table 5 shows the main process parameters and microstructure ratios of the controlled cooling of the steels in Examples 1-6; and Table 6 shows the microstructure and performance indicators of the steel plates in Examples 1-6.

[0051] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0052]

[0053] Note: Impurity elements in steel: P≤0.015%; S≤0.01%; O≤0.0050%; total amount of other impurity elements is less than 0.05%.

[0054] Table 2. Smelting and stacking process parameters of steel in the examples

[0055]

[0056] Table 3 Heating regime and high-pressure water descaling process parameters for the steel billets in the examples.

[0057]

[0058] Table 4 Rolling parameters of the steel in the examples

[0059]

[0060] Table 5. Main process parameters and microstructure ratios of steel controlled cooling in the embodiments of the present invention.

[0061]

[0062] Table 6. Microstructure and performance indicators of steel plates in embodiments of the present invention.

[0063]

[0064] Therefore, by employing the composition design and steelmaking continuous casting, controlled rolling, and controlled cooling scheme of this invention, the shortcomings of existing technologies are overcome, resulting in an economical X65M pipeline steel plate with excellent low-temperature toughness for gas pipelines with a thickness of 6-8.5mm and a pipe diameter of Φ914mm and below, and its manufacturing method. This steel plate has low cost, high strength, and high toughness, ensuring the safe operation of gas pipelines. The microstructure of the steel plate is a multiphase structure mainly composed of pearlite and polygonal ferrite, with polygonal ferrite accounting for more than 30%, M / A phase ratio less than 3%, and banded structure at level 3 and below. The proportion of grains with large angles above the grain boundaries in the microstructure is more than 60%. This steel plate has low cost, high strength, and high toughness, ensuring the safe operation of oil pipelines.

[0065] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing an ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness, characterized in that, Includes the following components by weight percentage: C: 0.04%~0.06%, Si: 0.16%~0.25%, Mn: 1.45%~1.60%, P≤0.015%, S≤0.01%, Nb: 0.03%~0.05%, Ti: 0.015%~0.025%, Al: 0.015%~0.04%, N: 0.0048%~0.006%. The weight percentage ratio of C, Si, Ti, Al, N, Nb, and Mn in the X65M pipeline steel plate conforms to: 3.40≤0.7(Ti / N)+0.5e （0.3Nb-Al） +0.75(2.4C-1.3Si+0.2Mn)≤4.00, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%; The pipeline steel plate has a transverse tensile yield strength of 470–510 MPa, a tensile strength of 570–630 MPa, an elongation of ≥25%, a transverse Charpy impact energy of ≥310 J at -30℃, and a DWTT transverse shear area SA of ≥95% at -30℃. The thickness of the pipeline steel plate is 6~6.5mm; In the metallographic structure of the pipeline steel plate, the volume fraction of polygonal ferrite is above 30%, the volume fraction of M / A phase is controlled within 3%, the remainder is pearlite, the banded structure is ≤ grade 3, and the proportion of large-angle grain boundaries with a crystal orientation difference of more than 15° in the metallographic structure is ≥ 60%. The preparation method of the ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness includes: KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, continuous casting, billet reheating, rolling, cooling and air cooling to room temperature; S1. The smelting raw materials are sequentially subjected to KR hot metal pretreatment, converter smelting, LF refining and RH vacuum degassing to obtain molten steel; S2. In the continuous casting process, the superheat is 8–15°C, and the casting speed is 0.9–1.3 m / min. The electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage is controlled at 150–200 A, the secondary cooling water volume is 1.8–2.2 L / kg, and the proportion of columnar crystals in the continuous casting billet is controlled at ≤15%. In the horizontal fan-shaped section, i.e., at the end of solidification, a light pressure is applied, and the reduction of the continuous casting billet is 5–10 mm. After the billet is cast, it is stacked and slowly cooled, with a stacking temperature ≥650°C and a slow cooling time ≥48 h, to obtain the cooled billet. The thickness of the continuous casting billet is 135–170 mm. S3. In the reheating of the billet, the cooled billet is passed sequentially through a preheating section, a heating section, and a soaking section before exiting the furnace to obtain a heated billet; wherein, the temperature range of the preheating section is 850-1100℃, the temperature range of the heating section is 1270-1295℃, the temperature range of the soaking section is 1250-1270℃, and the total furnace time of the heating section and the soaking section is 3.5-4.5h; S4. During the rolling and cooling processes, before rolling begins, the heated billet is descaled using high-pressure water after exiting the furnace. The rolling roll cycle is selected as 1500-2500 tons after roll change. Simultaneously, the cooling water in the front and rear roller tables of the rolling mill is shut off, and the flow rate of the rolling roll cooling water is controlled at 25-50 m³ / h. 3 The steel is rolled into a plate in two stages, in the recrystallization zone and the non-recrystallization zone respectively. The initial rolling temperature of the first stage is 1200-1220℃, the final rolling temperature of the first stage is 1030-1060℃, the number of rolling passes in the first stage is no more than 5, the reduction rate of each pass in the first and second passes is ≥35%, the thickness of the intermediate billet is 30-55.25mm, and the thickness of the intermediate billet is 5-6.5 times the thickness of the finished product. The initial rolling temperature of the second stage is 910–930℃, and the final rolling temperature of the second stage is 730–750℃. The number of rolling passes in the second stage shall not exceed 4, and the reduction rate of each pass in the first and second passes shall be ≥15%. After rolling, the steel is rapidly ejected at a speed of 6–7.5 m / s, and then cooled by laminar flow cooling at an initial cooling temperature of 650–680℃ and a final cooling temperature of 450–480℃, with a cooling rate of 5–10℃ / s.

2. The method for preparing the ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness according to claim 1, characterized in that, Step S1 includes: pretreating the smelting raw materials with KR molten iron to control the S content ≤0.01%, and then feeding them into the converter after slag removal; using the double slag method to remove P during converter smelting to control the P content ≤0.015%, and controlling the C content to be 0.04%~0.06% at the end of converter smelting; blowing argon gas for 20~25 minutes when tapping steel from the converter; and then sequentially performing LF refining and RH vacuum degassing, with RH vacuum degassing maintained for ≥30 minutes.

3. The method for preparing the ultra-thin, economical X65M pipeline steel plate with excellent low-temperature toughness according to claim 1, characterized in that, In step S4, the high-pressure water descaling time is 0.5 to 1 minute, and the pressure of the high-pressure water descaling is 10 to 15 MPa.

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