A thin gauge l450m pipeline steel plate based on strain design and a manufacturing method thereof

Abstract

The application discloses a kind of thin specification L450M pipeline steel plate based on strain design and manufacturing method thereof, belong to low carbon microalloy steel production technical field.The chemical composition of steel includes C 0.05%~0.08%, Si 0.15%~0.25%, Mn 1.55%~1.70%, P≤0.020%, S≤0.015%, Nb 0.02%~0.04%, Ti 0.008%~0.020%, Al 0.015%~0.04%, Cr 0.15%~0.25%, Mo 0.03%~0.06%, N 0.003%~0.006% by weight percentage, the balance is Fe and inevitable impurities, and the total amount of other impurity elements is less than 0.05%.The manufacturing method includes continuous casting and slab stacking slow cooling from molten steel smelting to slab heating→controlled rolling→relaxation+cooling→air cooling to room temperature.The steel plate has good toughness.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of low-carbon microalloyed steel production, and particularly relates to a thin-gauge L450M pipeline steel plate based on strain design and a manufacturing method thereof. BACKGROUND

[0002] In recent years, energy transportation pipeline construction has developed rapidly. In order to improve the transportation efficiency and operation safety, the requirements for the strength and toughness of the selected steel raw materials are higher and higher. At present, the major pipeline trunk lines at home and abroad basically adopt L485M and L555M steel grades, and the branch lines, urban pipeline networks and the like mostly adopt thin-gauge (9-15 mm) and small-diameter (Φ914 mm and below) L450M steel grade. When the pipeline region is in an extremely cold zone or passes through a zone, higher requirements are put forward for the low-temperature toughness, plastic deformation capacity and the like of the raw materials from the aspects of operation stability and safety, for example, the temperature for the low-temperature drop test is required to be lower, the Charpy impact energy is required to be higher, and the longitudinal uniform elongation of the steel pipe is also required to be higher. Therefore, these projects put forward higher individualized requirements, and it is far from enough to meet the API SPEC 5L specification. In addition, in the face of the severe market situation of the steel industry, user requirements and the pressure of manufacturing cost, it is particularly important to reduce the alloy and energy costs while ensuring product quality, to reduce resource consumption of the production line as much as possible. However, the steel plate after alloy reduction is usually produced by adopting a two-stage controlled rolling process, and in order to ensure the cumulative reduction rate in the finishing rolling stage and the static recrystallization of the intermediate blank, the intermediate blank needs to be kept at a relatively thick standby temperature, which leads to a longer standby time of the intermediate blank and a final rolling temperature controlled at 820℃ or even lower, resulting in a significant increase in rolling difficulty. First, the rolling mill load, steel plate temperature uniformity and shape control are all facing severe challenges, and second, the improvement of the steel plate performance mainly depends on the content of impurities such as phosphorus and sulfur in the steel, the content of alloy elements in the steel and the control of key process parameters in the production process such as smelting and rolling. Therefore, how to solve the problems existing in the production of the thin-gauge L450M pipeline steel plate based on strain design and reduce the alloy cost of the pipeline steel while ensuring its excellent performance index is the key to the development of the thin-gauge L450M pipeline steel based on strain design.

[0003] Comparison with prior art:

[0004] So far, there are few reports on the production of thin-gauge L450M pipeline steel plate designed for strain. Prior to the present application, the application number CN201010243241.0 discloses a kind of X65 pipeline steel and its production method. The weight percentage of the components of the patent is C 0.055%~0.090%, Si 0.15%~0.35%, Mn 1.50%~1.65%, P≤0.020%, S≤0.005%, Nb 0.040%~0.055%, V 0.040%~0.070%, Ti 0.010%~0.025%, N≤0.008%, Als 0.005%~0.060%. The composition of the steel grade contains a higher V, and the process adopts natural air cooling, which is low in efficiency, and cannot fully play the role of water-based alloy to reduce the cost and improve the strength and toughness, in addition, the steel grade also does not clearly indicate the specific strength and toughness, plastic deformation index of the product. The application number CN201110179945.0 discloses a kind of X65 pipeline steel with excellent low-temperature toughness and its manufacturing method. The weight percentage of the components of the 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 0.010%~0.040%, P≤0.018%, S≤0.005%, N≤0.006%. The composition of the steel grade contains V, which is high in alloy cost, and the process adopts laminar cooling, which cannot fully play the role of water-based alloy to reduce the cost and improve the strength and toughness, and does not meet the demand of strain design pipeline steel. The application number KR20020027013(A) discloses a kind of API-X65 pipeline with good aging performance and its manufacturing method (Steel product for stress relief heat treatment guaranteed API-X65 grade linepipes and method for manufacturing the same). The weight percentage of the components of the patent 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%, the composition of the steel grade contains V and Mo, which is high in cost and the product is insufficient in low-temperature toughness.

[0005] The steel disclosed in the above patent document has high strength and toughness, but they are either coiled or have high production cost, do not meet the technical requirements of the crossing belt (plastic deformation index-uniform elongation), and are mostly produced by two-stage controlled rolling, thus being not suitable for producing low-cost, thin-gauge, high-flatness L450M pipeline steel plate based on strain design. The technical scheme provided by the present application can effectively overcome the above-mentioned deficiencies, realize the production of thin-gauge L450M pipeline steel plate based on strain design with a thickness of 9-15 mm and a pipe diameter of Φ914 mm and below using a continuous casting billet with a thickness of 135-200 mm, and the steel plate has high strength, excellent low-temperature toughness and high plastic deformation resistance. SUMMARY

[0006] The present application aims to overcome the above-mentioned technical problems and deficiencies, and provides a thin-gauge L450M pipeline steel plate based on strain design and a manufacturing method thereof. The steel plate is a high-strength L450M hot-rolled steel plate for gas transmission pipelines, with a thickness of 9-15 mm and a pipe diameter of Φ914 mm or below. The problems of low rolling efficiency, high rolling resistance, poor uniformity of steel plate performance, low plastic deformation capacity and difficult control of plate shape are solved, and the steel plate has low cost, high strength and toughness, high plastic deformation resistance, and can ensure the safety of gas transmission pipelines.

[0007] To achieve the above-mentioned purposes, the technical scheme of the present application is as follows:

[0008] A thin-gauge L450M pipeline steel plate based on strain design, the chemical composition of which includes, by weight percentage: C 0.05%-0.08%, Si 0.15%-0.25%, Mn 1.55%-1.70%, P≤0.020%, S≤0.015%, Nb 0.02%-0.04%, Ti 0.008%-0.020%, Al 0.015%-0.04%, Cr 0.15%-0.25%, Mo 0.03%-0.06%, N 0.003%-0.006%, and the balance being Fe and unavoidable impurities, with the total amount of other impurity elements being less than 0.05%.

[0009] The effects of each main element in the chemical composition of the steel plate of the present application are as follows:

[0010] C: the most economical and basic strengthening element in steel, which has a significant effect on improving the strength of the steel through solid solution strengthening and precipitation strengthening, but increasing the C content has a negative impact on the plasticity, toughness and weldability of the steel. Therefore, the C content range is set to 0.05%-0.08% in the present application.

[0011] Mn: Mn strengthens steel through solid solution treatment, while 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 1.55%–1.70%.

[0012] Si (Si) plays a role in deoxidation in steelmaking and improving the strength of the matrix. 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. However, excessive Si will reduce the toughness of the heat-affected zone in the weld. Therefore, the Si content is set at 0.15% to 0.25% in this invention.

[0013] 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 0.02% to 0.04%, and with appropriate heating and rolling processes, a uniform multiphase microstructure dominated by bainite and polygonal ferrite can be obtained, with polygonal ferrite accounting for more than 65%, giving it good toughness.

[0014] N: In steel, nitrogen (N) has no other significant role besides forming fine TiN particles to refine austenite grains. Therefore, it needs to be kept at a low content level. The N content range selected in this invention is 0.003% to 0.006%.

[0015] 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 0.008% to 0.02%.

[0016] 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%.

[0017] Cr: It is a major element that can effectively improve hardenability, inhibit the formation of some ferrite and promote the formation of bainite. It plays an important role in controlling the phase transformation structure, and promotes the formation of polygonal ferrite, pearlite and acicular ferrite with a large number of dislocations in the grain in the medium and low temperature range, thereby improving the strength, plasticity and toughness of steel plates. The Cr content range selected in this invention is 0.15% to 0.25%.

[0018] Mo can significantly improve hardenability, increase strength, promote the transformation of microstructure at medium and low temperatures, and optimize the microstructure and properties of steel plates and heat-affected zones at welds. However, excessively high molybdenum content will increase production costs. Therefore, this invention controls the Mo content to be between 0.03% and 0.06%.

[0019] P and S are unavoidable impurity elements in steel, and their content should be as low as possible. However, due to considerations of smelting costs and processes, their content cannot be infinitely low. Therefore, this invention sets the upper limits for P and S content at 0.020% and 0.015%, respectively.

[0020] The present invention is based on strain design for high-strength L450M hot-rolled steel plates for gas pipelines with a target thickness of 9-15mm. The plates are produced on a medium-thickness reciprocating rolling mill using continuous casting billets with a thickness of 135-200mm and water as the cooling medium.

[0021] The objective of this invention is achieved through the following technical solution:

[0022] This invention provides a method for manufacturing thin-gauge L450M pipeline steel plates based on strain design, comprising: steel smelting → ladle refining (LF refining), RH vacuum degassing → continuous casting → billet heating → controlled rolling → relaxation + cooling → air cooling to room temperature; specifically including the following steps:

[0023] 1) Continuous casting of molten steel: smelted according to the following composition, the chemical composition by weight percentage including C 0.05%~0.08%, Si 0.15%~0.25%, Mn 1.55%~1.70%, P≤0.020%, S≤0.015%, Nb 0.02%~0.04%, Ti 0.008%~0.020%, Al 0.015%~0.04%, Cr 0.15%~0.25%, Mo 0.03%~0.06%, N 0.003%~0.006%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%. Molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. The superheating temperature during continuous casting is 9–15℃, and the casting speed is 0.7–1.0 m / min. In the fan-shaped section, strong cooling is used, with a total cooling water volume of 1000–1200 L / min in the first half and 950–1100 L / min in the second half. At the same time, light pressure is applied at the end of solidification, with a reduction of 8–15 mm for the continuously cast billet (strong cooling ensures the temperature gradient in the thickness direction of the billet, while light pressure promotes grain breakage in the core, ensuring the strength and toughness of the subsequent steel plate). After finishing, the billets are stacked for slow cooling, with a stacking temperature of not less than 650℃ and a slow cooling time of not less than 48 hours (to promote the diffusion of Mn and Cr elements and reduce their impact on microstructure and properties due to component segregation).

[0024] 2) Billet Heating: The billet is fed into a walking beam furnace for heating, passing through a preheating section, a heating section, and a soaking section before exiting the furnace. The preheating section has a temperature range of 950–1100℃ (to promote the rapid and complete dissolution and diffusion of Nb, Cr, and Ti carbides and nitrides into the matrix), the heating section has a temperature range of 1210–1230℃, and the soaking section has a temperature range of 1100–1125℃. The total furnace time for the heating and soaking sections is controlled at 2.5–3.5 hours, ensuring that the temperature difference between the upper and lower surfaces of the billet is within 15℃. (The heating section provides high-temperature heating to ensure temperature uniformity in all parts of the billet, further promoting the diffusion of alloying elements Mn, Cr, and C, reducing their impact on microstructure and properties due to component segregation, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface. Combined with the billet composition, reducing the heating temperature of the soaking section reduces energy consumption and also inhibits the impact of coarsening of the original austenite grains on DWTT performance.)

[0025] 3) Controlled rolling: The billet adopts a transverse and longitudinal rolling mode. In the transverse rolling stage, the reduction rate of each of the first two passes is greater than 20%, and the first pass of rolling is sprayed with descaling water from the mill for 1-1.5 minutes at a pressure of 20-25 MPa. (The first two passes of rolling should maximize the capacity of the mill and use a large reduction rate to promote the dynamic re-crystallization of austenite, refine the original austenite grains, and use high-pressure water descaling from the mill to refine the grain structure from the surface to the core of the steel plate.) The austenite grains are evenly distributed, improving the strength and toughness of the steel plate. In the longitudinal rolling stage, the reduction rate of each of the first three passes is greater than 25%, and the reduction rate of the last pass is less than 5%. The final rolling temperature of the steel plate is 780-810℃. (The first three passes of the longitudinal rolling stage use a large reduction rate to further refine the austenite grains and improve the core structure. In the later stage of rolling, due to the temperature drop of the steel plate and the increase in deformation resistance, the last pass uses a small reduction rate to flatten the shape of the steel plate and reduce the internal stress of the steel plate.)

[0026] 4) Relaxation + Cooling: After rolling, the steel plate undergoes relaxation and pre-straightening (to ensure the initial plate shape upon immersion in water, while controlling the immersion temperature to ensure the ferrite content is not less than 65%). Laminar flow cooling is then employed, with an initial cooling temperature range of 610–630℃ and a final cooling temperature range of 370–400℃, at a cooling rate controlled at 10–15℃ / s. (Slow cooling is used to ensure the bainite content is below 30%, while reducing pearlite formation, suppressing the formation of wide banded structures, and ensuring the low-temperature DWTT performance and plasticity of the steel plate). After the steel plate exits controlled cooling, side spraying and air purging are activated, with side spraying pressure and water flow rate of 5–10 MPa and 35–50 m³ / s, respectively. 3 / h, the air purging pressure is 5-10MPa, followed by hot straightening.

[0027] 5) Air cool to room temperature.

[0028] Furthermore, in step 1), the raw material is pretreated with KR molten iron to control the S content to be below 0.015%, and then enters the converter after slag removal; during converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be between 0.05% and 0.08% at the end of converter smelting. Argon gas is blown for 15 to 20 minutes when tapping the steel; then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes; after that, continuous casting is carried out.

[0029] Furthermore, during the converter smelting process, a double-slag method is used for phosphorus removal.

[0030] Furthermore, in the continuous casting process, the first half is section 1-4, and the second half is section 5-8.

[0031] Furthermore, in step 3), before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with the descaling machine pressure being 15-25 MPa.

[0032] Furthermore, in step 4), the number of controlled cooling manifold groups opened is 3 to 6, and the water flow rate per manifold is 120 to 150 m³. 3 / h.

[0033] Furthermore, in step 4), hot straightening is performed in three stages: the inlet roller position is -1.5mm to -2.6mm, the outlet roller position is -3.3mm to -4.1mm, and the straightening force is between 2500KN and 3000KN (by setting appropriate roller gaps and straightening forces, the straightened steel plate is ensured to be straight and have a good shape).

[0034] By adopting the above-mentioned composition and steelmaking continuous casting, controlled rolling and controlled cooling scheme, the shortcomings of the existing technology have been overcome, and the production and application of L450M pipeline steel plate for gas transmission pipelines with thin specifications (9-15mm) and pipe diameters below Φ914mm based on strain design have been realized. The microstructure of the steel plate is a multiphase microstructure mainly composed of bainite and polygonal ferrite, of which polygonal ferrite accounts for more than 65%.

[0035] The beneficial effects of this invention are:

[0036] 1. After deep desulfurization pretreatment of KR hot metal, the slag is thoroughly removed. The converter employs a double-slag method for P removal, resulting in lower P and S content in the billet. Controlling the argon blowing time and maintaining the RH vacuum degassing time overcomes defects such as center segregation, inclusions, and excessive H and O content in the billet caused by high Mn, Cr, and C content, thus improving the plasticity and toughness of pipeline steel. Reducing superheat and continuous casting speed improves macroscopic segregation in the continuously cast billet, reduces the spacing of secondary dendrite arms in the solidification structure, and helps reduce billet segregation and internal structural defects. Strong cooling is used in the fan-shaped section to ensure a temperature gradient along the billet thickness, while light pressure promotes core grain breakage, ensuring the strength and toughness of the subsequent steel plate. Simultaneously, the billets are stacked and slowly cooled after being taken off the line, with the stacking temperature not exceeding the slow cooling time limit, which helps promote the diffusion of Mn and Cr elements and mitigates their impact on microstructure and properties due to component segregation.

[0037] 2. Billet Heating: The temperatures and times of the billet in the preheating, heating, and soaking zones are limited to ensure that Nb, Cr, and Ti carbides and nitrides dissolve rapidly and fully in the matrix and diffuse sufficiently. Simultaneously, the heating zone provides high-temperature heating to ensure temperature uniformity across the billet, further promoting the diffusion of alloying elements Mn, Cr, and C, mitigating their impact on microstructure and properties due to compositional segregation, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface. Combined with the billet composition, reducing the heating temperature in the soaking zone decreases energy consumption and also suppresses the impact of initial austenite grain coarsening on DWTT performance.

[0038] 3. A transverse and longitudinal rolling process is adopted. The first few passes in the transverse and longitudinal rolling stages maximize the mill's capacity, employing a high reduction rate to promote dynamic austenite recrystallization and refine the original austenite grains. High-pressure water descaling in the mill ensures a uniform microstructure distribution from the surface to the core of the steel plate, improving its strength and toughness. Furthermore, this rolling process does not employ the traditional TMCP process, eliminating the need for intermediate billet warming, which also improves rolling efficiency. The water immersion temperature of the rolled steel plate is controlled to ensure that the ferrite content is not less than 65% before immersion. A slow cooling rate is used to ensure that the bainite content is below 30%, while simultaneously reducing pearlite formation and suppressing the formation of wide banded structures. This ensures the low-temperature DWTT performance and plasticity of the steel plate. Side spraying, air purging, and hot straightening facilitate control of the steel plate shape, improve the uniformity of steel plate performance, reduce the likelihood of head and tail shape problems, and save on subsequent cold straightening equipment investment costs.

[0039] 4. This invention reduces alloy costs through simple composition design and achieves the production and application of a thin-gauge L450M pipeline steel plate based on strain design by controlling the steelmaking, continuous casting, heating, controlled rolling and cooling, and hot straightening processes. The steel plate has a thickness of 9-15mm and a pipe diameter of Φ914mm or less. The microstructure of the steel plate is a multiphase structure mainly composed of bainite and polygonal ferrite, with polygonal ferrite accounting for more than 65%. The steel plate has good low-temperature toughness and plasticity. Specific performance indicators are as follows: Transverse: tensile yield strength between 480 and 520 MPa, tensile strength between 570 and 630 MPa, yield ratio less than 0.85, elongation ≥25%. Transverse Charpy impact energy at -40℃ ≥150J, transverse shear area (SA) at -20℃ not less than 90%, longitudinal: uniform elongation ≥10%. Detailed Implementation

[0040] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0041] A thin-gauge L450M pipeline steel plate based on strain design has the following chemical composition by weight percentage: C 0.05%–0.08%, Si 0.15%–0.25%, Mn 1.55%–1.70%, P ≤0.020%, S ≤0.015%, Nb 0.02%–0.04%, Ti 0.008%–0.020%, Al 0.015%–0.04%, Cr 0.15%–0.25%, Mo 0.03%–0.06%, N 0.003%–0.006%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%. The steel plate is manufactured with a thickness of 9-15 mm, using continuously cast billets with a thickness of less than 250 mm on a medium-thickness reciprocating rolling mill, with water as the cooling medium.

[0042] The manufacturing method of the aforementioned thin-gauge L450M pipeline steel plate based on strain design includes steel smelting → ladle refining (LF refining), RH vacuum degassing → continuous casting → billet heating → controlled rolling → relaxation + cooling → air cooling to room temperature; specifically, it includes the following steps:

[0043] 1) Steel smelting and continuous casting: smelting according to the above composition. The raw materials undergo KR hot metal pretreatment to control the sulfur content to be below 0.015%, and are then fed into the converter after slag removal. During converter smelting, the phosphorus content is controlled to be ≤0.02%, and the carbon content is controlled to be 0.05-0.08% at the end of converter smelting. Argon gas is blown for 15-20 minutes during tapping. This is followed by LF refining and RH vacuum degassing, with the RH vacuum maintained for at least 30 minutes. Continuous casting then takes place, with a superheat of 9-15℃ and a billet pulling speed of 0.7-1.0 m / min. In the sector section, strong cooling is used, with a total cooling water volume of 1000-1200 L / min for sections 1-4 and 950-1100 L / min for sections 5-8. Simultaneously, light reduction is applied at the end of solidification, with a billet reduction of 8-15 mm. After casting, the billets are stacked for slow cooling, with a stacking temperature not lower than 650℃ and a slow cooling time not lower than 48 hours.

[0044] 2) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature range of the preheating section is 950-1100℃, the temperature range of the heating section is 1210-1230℃, and the temperature range of the soaking section is 1100-1125℃. The total time in the furnace for the heating and soaking sections is controlled at 2.5-3.5 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃.

[0045] 3) High-pressure water descaling and controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with a descaling machine pressure of 15-25 MPa; a transverse and longitudinal rolling mode is adopted. In the transverse rolling stage, the reduction rate of each pass in the first two rolling passes is greater than 20%, and the first rolling pass is sprayed with descaling water from the rolling mill for 1-1.5 minutes at a pressure of 20-25 MPa; in the longitudinal rolling stage, the reduction rate of each pass in the first three rolling passes is greater than 25%, and the reduction rate of the last pass is less than 5%, with a final rolling temperature of 780-810℃.

[0046] 4) Relaxation + Cooling: After rolling, the steel plate is relaxed and pre-straightened before being put into the water (to ensure the initial plate shape upon immersion in the water, while controlling the immersion temperature to ensure that the ferrite content of the steel plate is not less than 65% before immersion). Laminar flow cooling is then employed, with an initial cooling temperature range of 610–630℃ and a final cooling temperature range of 370–400℃. The number of controlled cooling manifolds is 3–6, and the water flow rate per manifold is 120–150 m³. 3The cooling rate is controlled at 10~15℃ / s per hour to ensure the bainite content is below 30%. After the steel plate exits controlled cooling, the side spray and air purging are activated, with side spray pressure and water flow rate of 5~10MPa and 35~50m³ / h, respectively. 3 / h, the air blowing pressure is 5~10MPa, and then hot straightening (three straightening) is performed. The position of the inlet roller is -1.5mm~-2.6mm, the position of the outlet roller is -3.3mm~-4.1mm, and the straightening force is between 2500KN and 3000KN.

[0047] 5) Air cool to room temperature.

[0048] Examples 1-6

[0049] Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the smelting, continuous casting, and stacking process of the steel in the examples; Table 3 shows the heating process of the billet and the high-pressure water descaling process before rolling of the continuously cast billet of the steel in the examples; Table 4 shows the rolling and partial cooling process of the steel in the examples; Table 5 shows the main process parameters of controlled cooling of the steel in the examples; Table 6 shows the properties of the steel plates in the examples.

[0050] Table 1 Chemical composition (wt, %) of the steel in the examples

[0051]

[0052] Note: Impurity elements in steel: P≤0.02%; S≤0.015%; total amount of other impurity elements less than 0.05%.

[0053] Table 2. Process Regulations for Steel Smelting, Continuous Casting, and Stacking in Examples

[0054]

[0055] Table 3 Heating regime of steel billets and high-pressure water descaling process before rolling of continuously cast billets in the examples

[0056]

[0057] Table 4 Rolling and Partial Cooling Processes of the Steel in the Examples

[0058]

[0059] Table 5. Main process parameters for controlled cooling of steel in the examples

[0060]

[0061] Table 6 Performance of the steel plates in the examples

[0062]

[0063] Therefore, compared with the prior art, this invention reduces alloy costs through simple composition design and obtains a highly efficient production and application of L450M pipeline steel plates for gas transmission pipelines with a thickness specification (9-15mm) and a pipe diameter of Φ914mm or less by controlling the steelmaking, continuous casting, heating, controlled rolling and cooling and hot straightening processes. The steel plate has a multiphase structure mainly composed of bainite and polygonal ferrite, with polygonal ferrite accounting for more than 65%. The steel plate has good low-temperature toughness and plasticity.

[0064] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A thin-gauge L450M pipeline steel plate based on strain design, characterized in that, Its chemical composition by weight percentage includes: C 0.05%–0.08%, Si 0.15%–0.25%, Mn 1.55%–1.70%, P≤0.020%, S≤0.015%, Nb 0.02%–0.04%, Ti 0.008%–0.020%, Al 0.015%–0.04%, Cr 0.15%–0.25%, Mo 0.03%–0.06%, N 0.003%–0.006%, with the balance being Fe and unavoidable impurities, and the total amount of other impurity elements being less than 0.05%. The manufacturing method of the thin-gauge L450M pipeline steel plate based on strain design includes the following steps: 1) Steelmaking to continuous casting and billet stacking slow cooling: Molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting; the superheat of continuous casting is 9-15℃, and the billet pulling speed is 0.7-1.0m / min; in the sector section, strong cooling is adopted, with a total cooling water volume of 1000-1200L / min in the first half and 950-1100L / min in the second half, while light reduction is applied at the end of solidification, with a billet reduction of 8-15mm; after the billet is removed from the line, it is stacked for slow cooling, with a stacking temperature of not less than 650℃ and a slow cooling time of not less than 48h; 2) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature of the preheating section is 950-1100℃, the temperature of the heating section is 1210-1230℃, and the temperature of the soaking section is 1100-1125℃. The total time in the furnace for the heating and soaking sections is controlled at 2.5-3.5 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃. 3) Controlled rolling: The rolling mode of transverse and longitudinal rolling is adopted. In the transverse rolling stage, the reduction rate of each pass of the first two rolling passes is greater than 20%, and the first rolling pass is sprayed with descaling water for 1 to 1.5 minutes and the pressure is 20 to 25 MPa. In the longitudinal rolling stage, the reduction rate of each pass of the first three rolling passes is greater than 25%, and the reduction rate of the last pass is less than 5%. The final rolling temperature of the steel plate is 780 to 810℃. 4) Relaxation + Cooling: After rolling, the steel plate is relaxed and placed in pre-straightening, followed by laminar flow cooling. The initial cooling temperature is 610-630℃, the final cooling temperature is 370-400℃, and the cooling rate is controlled at 10-15℃ / s; then hot straightening is performed. 5) Air cool to room temperature.

2. The thin-gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that, The thickness of the steel plate is 9-15mm.

3. The thin-gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that, The steel plate has a transverse tensile yield strength of 480-520 MPa, a tensile strength of 570-630 MPa, a yield strength ratio of less than 0.85, and an elongation of ≥25%; a transverse Charpy impact energy of ≥150 J at -40℃, a transverse shear area of ​​not less than 90% at -20℃ DWTT, and a longitudinal uniform elongation of ≥10%.

4. The thin-gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that, The thickness of the cast billet is 135-200mm; the steel plate is obtained by rolling the cast billet on a medium-thickness reciprocating rolling mill.

5. The thin-gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that, In step 1), the raw material is pretreated with KR molten iron to control the S content to be less than 0.015%, and then enters the converter after slag removal. During converter smelting, the P content is controlled to be ≤0.02%, and the C content is controlled to be 0.05-0.08% at the end of converter smelting. Argon gas is blown for 15-20 minutes when tapping the steel. Then, LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes. After that, continuous casting is carried out.

6. The thin-gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that, During the converter smelting process, the double-slag method is used for phosphorus removal; During the continuous casting process, the first half is sections 1-4, and the second half is sections 5-8.

7. The thin-gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that, In step 3), before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with a descaling machine pressure of 15-25 MPa; after the steel plate exits controlled cooling, the side spraying and air purging are started, with the side spraying pressure and water volume being 5-10 MPa and 35-50 m³ / h, respectively. 3 / h, the air purging pressure is 5-10MPa, followed by hot straightening.

8. The thin-gauge L450M pipeline steel plate based on strain design according to claim 1, characterized in that, In step 4), the cooling medium for laminar flow cooling is water; the number of controlled cooling manifold groups is 3 to 6, and the water flow rate per manifold is 120 to 150 m³. 3 / h; Hot straightening involves three straightening stages: the inlet roller position is -1.5mm to -2.6mm, the outlet roller position is -3.3mm to -4.1mm, and the straightening force is 2500kN to 3000kN.

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