A 1000mpa grade marine environment fatigue-resistant steel plate for conveying mineral pipelines and a production method thereof

Abstract

The application belongs to the field of metal materials, and provides a 1000MPa-grade marine environment fatigue-resistant mine pipeline steel plate and a production method thereof.The composition of the steel plate is as follows in terms of percentage by weight: C: 0.13%-0.17%, Si: 0.31%-0.50%, Mn: 0.96%-1.20%, P: ≤0.015%, S: ≤0.001%, Cr: 0.70%-1.10%, Mo: 0.25%-0.45%, Al: 0.010%-0.050%, Nb: 0.08%-0.12%, V: 0.11%-0.20%, Ti: 0.005%-0.025%, Ni: 0.31-0.50%, Ce: 0.01-0.06%, and the balance of Fe and inevitable impurities.The production method comprises the following steps: heating, rolling, accelerated cooling, and slow cooling.The steel plate produced by the application has good strength and toughness, good corrosion resistance, and excellent fatigue resistance.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and in particular relates to a 1000MPa grade steel plate for marine environment fatigue-resistant ore transport pipelines and its production method. Background Technology

[0002] The deep sea crust contains the vast majority of ultra-high-grade metallic minerals such as manganese, cobalt, copper, and nickel, with their distribution, grade, and reserves far exceeding those on land. Deep-sea mining will become an important part of future mineral resource development. Mineral transportation is a crucial link in deep-sea mining. Pipeline-lift deep-sea mining systems have proven to be the most commercially viable after multiple sea trials. Deep-sea mineral resource development faces extremely complex and harsh operating environments, including typhoons, giant waves, and internal waves. In particular, deep-sea mining involves transporting minerals from underwater to the surface via pipelines. These pipelines connect to surface platforms and seabed ore collection systems at both ends, while the middle section hangs suspended in the deep sea. They must withstand not only internal wear and tear from mineral transportation but also the effects of external ocean currents, wave action, and the drag of their own weight. This necessitates that deep-sea mining pipelines possess not only wear resistance and seawater corrosion resistance but also excellent fatigue resistance.

[0003] Patent application number 202310647877.9 discloses a "Fatigue-Resistant Marine Riser Steel and its Preparation Method," with the following chemical composition: C: 0.04%–0.07%, Si: 0.20%–0.40%, Mn: 1.00%–1.60%, Nb: 0.02%–0.06%, V: 0.030%–0.055%, Ti: 0.01%–0.02%, Cr: 0.10%–0.30%, Ni: 0.10%–0.30%, Mo: 0.05%–0.25%, P: <0.005%, S: <0.005%, with the remainder being Fe and unavoidable impurities. The microstructure is characterized by ultrafine-grained ferrite and acicular ferrite. Its application is for transporting oil and natural gas media; however, due to its low C content, it has poor wear resistance and is unsuitable for transporting high-hardness media such as minerals.

[0004] Patent application number 201811584851.X discloses a "High-Strength, Tough, and Fatigue-Resistant Nanoprecipitate Reinforced Martensite-Austenite Duplex Steel and its Preparation Method," with the following chemical composition: C: 0.06%–0.20%; Si: 0.05%–0.20%; Mn: 2.50%–5.00%; P≤0.01%; S≤0.01%; Cu: ≤1.00%; Ni: ≤2.00%; Mo: 0.20%–0.50%; Cr: 1.00%–1.50%; V: 0.10%–0.80%; the remainder being Fe and unavoidable impurities. Tempered martensite + reverse austenite is obtained through quenching and tempering heat treatment, resulting in excellent fatigue resistance and corrosion resistance. However, the high content of alloying elements, especially Mn exceeding 2.5%, not only significantly increases the alloy cost but also negatively impacts the weldability of the steel.

[0005] Patent application number 202410698271.2 discloses "An Ultra-fine Bainitic Wear-resistant Steel NM300 and Its Production Method," with the following chemical composition: C 0.13%–0.15%, Si 0.4%–0.6%, Mn 1.3%–1.5%, Alt 0.3%–0.5%, Nb 0.01%–0.03%, Ti 0.01%–0.03%, B 0.001%–0.004%, with the balance being Fe and trace impurities. By adding B and achieving a cooling rate ≥80℃ / s, an ultra-fine bainitic + retained austenitic structure is obtained, enabling steel plates with a thickness of 2-8mm to achieve good wear resistance. However, this design lacks resistance to seawater corrosion, and the steel plate wall thickness is too thin, resulting in poor crush resistance. It is unsuitable for the high-pressure environment of deep sea and the highly abrasive environment of transporting seawater-containing minerals, and cannot meet the stringent service conditions of deep-sea mineral transport.

[0006] Patent application number 201710182924.1 discloses a "Superior High-Strength Steel Plate with Excellent Seawater Corrosion Resistance, Fatigue Resistance, and Environmental Brittleness Resistance, and its Manufacturing Method," with the following chemical composition: C 0.025%–0.055%, Si ≤0.10%, Mn 0.45%–0.85%, P ≤0.013%, S ≤0.0050%, Cr 11%–16%, Mo 0.85%–1.25%, Ni 4.50%–6.50%, Nb 0.05%–0.08%, Ti 0.008%–0.018%, Ca 0.0015%–0.0040%, with the balance being iron and unavoidable impurities. Through a high-Cr, high-Mo, and high-Ni composition design, an ultra-high-strength steel plate with excellent fatigue resistance and environmental brittleness resistance is obtained. However, an alloy content exceeding 16% not only increases alloy costs but also negatively impacts the formability and weldability of the steel plate.

[0007] In summary, the current production of steel for ore conveying risers does not involve a design that matches the material's comprehensive properties of wear resistance and fatigue resistance to the characteristics of mineral transportation and marine service conditions. It is not suitable for the highly abrasive environment of mineral transportation and cannot meet the stringent service conditions of deep-sea mineral transportation. Summary of the Invention

[0008] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a 1000MPa grade marine environment fatigue-resistant ore transport pipeline steel plate with good toughness, wear resistance and excellent fatigue resistance, as well as its production method.

[0009] The objective of this invention is achieved as follows:

[0010] A 1000MPa grade marine environment fatigue-resistant ore transport pipeline steel plate, the composition of which is as follows by weight percentage: C: 0.13%~0.17%, Si: 0.31%~0.50%, Mn: 0.96%~1.20%, P≤0.015%, S≤0.001%, Cr: 0.70%~1.10%, Mo: 0.25%~0.45%, Al: 0.010%~0.050%, Nb: 0.08%~0.12%, V: 0.11%~0.20%, Ti: 0.005%~0.025%, Ni: 0.31~0.50%, Ce: 0.01~0.06%, with the balance being Fe and unavoidable impurities.

[0011] Furthermore, the Cr+Mo content in the steel plate is ≥1.10%.

[0012] Furthermore, the Nb / Ce ratio in the steel plate is ≥1.8.

[0013] Furthermore, the V / Ce ratio in the steel plate is ≥2.2.

[0014] Furthermore, the microstructure of the steel plate is a mixed structure of lower bainite, lath martensite, and nanoscale V-containing precipitates, which are expressed as follows by mass percentage: lower bainite 30%~50%; nanoscale precipitates 1.0%~2.0%, of which more than 90% of the precipitates have a size ≤40nm.

[0015] Furthermore; the steel plate has a yield strength of 1000~1100MPa, a tensile strength of 1150~1250MPa, and an elongation of ≥18%; the steel plate has an impact energy of ≥110J at -20℃. 7 Cycle fatigue strength ≥250MPa, hardness ≥370HBW.

[0016] The rationale for the design of the components in this invention is as follows:

[0017] Carbon (C): A fundamental strengthening element in steel, C is the main element ensuring strength and hardness in this invention. Adding an appropriate amount of C can significantly improve the strength and hardness of the steel plate. C can also combine with V and Nb to form fine granular precipitates, further enhancing the strength and toughness of the steel. However, excessive C is detrimental to the welding, toughness, and plasticity of the steel plate. Therefore, considering factors such as cost and performance, this invention controls the C content to be between 0.13% and 0.17%.

[0018] Si: Si is an essential element for deoxidation during steelmaking. It has strong solid solution properties in steel, which can improve the strength and hardness of steel, thereby improving its wear resistance. Adding an appropriate amount of Si to steel can promote the finer and more dispersed distribution of carbides in the matrix and enrichment around the carbides, inhibiting cementite precipitation, hindering carbide growth, improving the fatigue resistance of steel, and further improving its wear resistance. However, a high Si content is detrimental to low-temperature toughness, reducing the low-temperature toughness of the steel plate, worsening the toughness of the steel, and reducing its plasticity. In this invention, the Si content is controlled at 0.31~0.50%.

[0019] Mn (manganese) is a common strengthening element in steel. It can dissolve in large quantities in the Fe matrix, increasing strength through solid solution strengthening. Manganese can lower the critical cooling rate of steel, promote martensite formation, significantly improve hardenability, and significantly lower the brittle transition temperature of steel, improve impact toughness, and refine the microstructure of steel, making it an important strengthening and toughening element. However, excessive Mn content can easily lead to segregation in steel, which has adverse effects on the fatigue and corrosion properties of steel. Taking all factors into consideration, this invention controls the Mn content to be in the range of 0.96%~1.20%.

[0020] Cr and Mo can improve the hardenability and bainitrification tendency of steel, effectively increasing its strength, hardness, and fatigue strength, resulting in good wear resistance and fatigue resistance. Adding a certain amount of Cr and Mo to steel can also effectively fill the voids formed by pitting corrosion caused by chloride ions in marine environments, forming a dense protective film that prevents further pitting corrosion and thus provides corrosion resistance in marine environments. In this invention, the Cr content is controlled within the range of 0.70%~1.10%, and the Mo content is controlled within the range of 0.25%~0.45%, preferably; Cr+Mo≥1.10%.

[0021] Nitrogen (Nb) is a grain-refining element. When heated, undissolved Nb carbon and nitride particles are distributed along the austenite grain boundaries, hindering austenite grain growth during heating. It effectively delays the recrystallization of deformed austenite, preventing austenite grain growth, refining ferrite grains, improving the impact toughness of steel, and lowering its brittle transition temperature. Nb can also increase the solid solution content of rare earth elements such as Ce in steel. When Nb and Ce are added together, Ce can be fully dissolved in the steel, thereby improving its corrosion resistance. An appropriate amount of Nb can further increase the non-recrystallization temperature window of the steel, enabling non-recrystallization rolling at higher temperatures and reducing the rolling load on the mill. In this invention, the Nb content is controlled at 0.08%~0.12%, preferably Nb / Ce ≥ 1.8.

[0022] Vanadium (V) is a strong carbide-forming element with a strong affinity for C and N. Vanadium carbides are highly dispersed and extremely stable, which can improve the strength and toughness of steel by refining its microstructure and grain size, while also improving fatigue resistance. V combines with C and N to form second-phase particles that precipitate during welding, ensuring the strength of the heat-affected zone (HAZ) and significantly reducing HAZ softening. Under certain C content conditions, the combined addition of V and Nb can form dispersed nanoscale carbide particles in the steel, significantly improving its strength and hardness without deteriorating toughness, thereby enhancing its wear resistance. V can also increase the solid solution content of rare earth elements such as Ce in steel, thus improving its corrosion resistance. In this invention, its content is controlled at 0.11–0.20%, and V / Ce ≥ 2.2.

[0023] Ti is a strong nitrogen-containing element. Adding trace amounts of Ti to Al-containing steel can significantly reduce the corrosion rate, and Ti preferentially combines with nitrogen in the steel, reducing the amount of AlN and improving weldability. However, when the Ti content exceeds a certain value, the TiN particles coarsen, causing a deterioration in low-temperature toughness. Therefore, this invention selects a Ti content of 0.010% to 0.025%.

[0024] Al: Commonly used as a deoxidizer in steel, insufficient deoxidation leads to the formation of oxides from easily oxidized elements such as Ti; excessive aluminum content increases alumina inclusions, deteriorating the cleanliness of the steel and negatively impacting fatigue resistance. Therefore, this invention controls the Al content to be in the range of 0.010% to 0.050%.

[0025] Ni (Ni): It can improve the strength of steel, lower the critical cooling temperature, facilitate microstructure control and grain refinement, and improve low-temperature toughness and fatigue resistance. However, Ni is expensive and should not be added in excess. This invention controls the Ni content to 0.31%-0.50%.

[0026] Ce preferentially reacts with O and S during steelmaking to form stable oxides and sulfides, transforming strip-shaped MnS inclusions into spherical ones. It also promotes the full flotation of these compounds in molten steel, facilitating inclusion removal and reducing the formation of large inclusions. This mitigates the cutting effect of irregular and large inclusions on the matrix, purifying the molten iron and significantly improving fatigue resistance. During steel solidification, Ce also acts as a nucleation core, increasing the number of crystal nuclei and refining the grains, further enhancing the steel's strength, toughness, and fatigue resistance. This invention controls the Ce content to be 0.01%-0.06%.

[0027] P and S: As impurity elements, phosphorus and sulfur significantly deteriorate the toughness, plasticity and weldability of steel, so their content in steel should be reduced as much as possible. In this invention, the content of P and S is required to be controlled at P≤0.015% and S≤0.001%, respectively.

[0028] The second technical solution of the present invention is to provide a 1000MPa grade fatigue-resistant steel plate for marine mining pipelines and its production method, including: heating, rolling, accelerated cooling, and slow cooling;

[0029] Heating: The slab adopts a multi-stage stepped temperature heating method, in which the high temperature heating section is 1200~1250℃, the soaking temperature is 1180~1220℃, and the time of the high temperature heating section + soaking section is not less than 120min.

[0030] The heating process is designed primarily to ensure the solid solution of elements such as Mn, Cr, Mo, Ni, Cu, Nb, and V, enabling them to play a role in subsequent rolling and accelerated cooling. At the same time, it effectively controls austenite grain growth. The heating time ensures heating effect and temperature uniformity.

[0031] Rolling: Steel plate rolling is carried out in two stages: roughing and finishing. The finishing temperature of the roughing stage is ≥1050℃, and the single-pass reduction rate of the last two passes of roughing is ≥14%, so as to fully break down and refine the original austenite grains. The intermediate billet thickness of the finishing stage is 3.5~4.0t, where t is the thickness of the finished steel plate (12~20mm). The initial rolling temperature is 850~900℃, the single-pass reduction rate of the first three passes of finishing is ≥13%, and the final rolling temperature is 780~820℃.

[0032] The rolling temperature design primarily considers minimizing deformation resistance during roughing and finishing rolling to achieve a greater degree of deformation, ensuring the rolling effect penetrates fully in the thickness direction, resulting in excellent thickness-direction microstructure uniformity, thereby improving fatigue resistance. Simultaneously, it promotes the formation of nanoscale precipitates, pinning grain boundaries and dislocations, thus refining the grains. A larger single-pass reduction rate is used to fully break and flatten the austenite grains, combined with a relatively low finishing rolling temperature to increase the dislocation density in the steel. This grain refinement and dislocation strengthening enhance strength and toughness.

[0033] Accelerated cooling: After the steel plate is rolled, accelerated cooling is carried out in two stages. The first stage begins at a cooling temperature of 760~800℃, with a cooling rate of 5~10℃ / s and a cooling time of 8~10s. The purpose of this stage is to suppress the transformation of austenite to ferrite and to obtain a granular bainite structure in the steel plate with an appropriate cooling rate. The second stage of cooling has no significant time or distance interval from the first stage, with a cooling rate increased to 15~20℃ / s and a final cooling temperature of 360~430℃.

[0034] Slow cooling: After the steel plates have undergone accelerated cooling, they are slowly cooled using a stacking method at a cooling rate of ≤0.2℃ / s until they reach room temperature. The purpose is to slowly release the internal stress in the steel and prevent cracking caused by a sharp increase in stress due to the drastic changes in the steel's microstructure caused by the high C content and the presence of hardenable elements such as Cr and Mo after accelerated cooling.

[0035] The aim is to suppress the formation of lamellar martensite by using appropriate cooling rates and final cooling temperatures, while promoting the formation of short, lath-like martensite, ultimately obtaining a mixed microstructure of lower bainite, lath-like martensite, and nanoscale V-containing precipitates. The lower bainite content is 30%–50%, and the proportion of nanoscale precipitates is 1.0%–2.0%, with over 90% of the precipitates having a size ≤40 nm and exhibiting a dispersed distribution. The lower bainite and short, lath-like martensite in the final microstructure effectively coordinate strain, effectively addressing the traction and tension of the steel pipe caused by the movement of the floating body, reducing stress concentration caused by cyclic deformation, and effectively resisting fatigue crack initiation. They also effectively absorb the energy from crack initiation, hindering further crack propagation and improving crack arrest performance. The lath-like martensite improves the strength and wear resistance of the steel plate. The martensite, as a hard phase surrounded by bainite, not only increases the strength of the steel but also does not deteriorate its fatigue resistance. The dispersed nanoscale V-containing precipitates, acting as hard phase particles, not only significantly improve the wear resistance of steel but also maintain their dispersed precipitation state during welding, ensuring the strength of the weld heat-affected zone, significantly improving heat-affected zone softening, enhancing the overall performance uniformity of the steel pipe, and thus improving its fatigue fracture resistance. The cooled steel plates are stacked and slowly cooled to room temperature to fully release the internal stress of the steel plates.

[0036] The beneficial effects of this invention are as follows:

[0037] 1) This invention, based on C and Mn elements, adds appropriate amounts of alloying elements such as Cr, Mo, Ni, Nb, and V. Through the interaction between these elements and with suitable heating, rolling, and cooling processes, a composite microstructure of lower bainite + lath martensite + nanoscale V-containing precipitates is finally obtained. Compared with existing technologies, this invention employs targeted composition and microstructure design to improve the fatigue and wear resistance of steel plates, resulting in steel plates with good strength, toughness, wear resistance, and excellent fatigue resistance.

[0038] 2) The steel plate possesses excellent comprehensive mechanical properties, with a yield strength of 1000-1100 MPa, tensile strength of 1150-1250 MPa, and elongation ≥18%. The steel plate has an impact energy of ≥110 J at -20℃, a fatigue strength of ≥250 MPa after 107 cycles, and a hardness of ≥370 HBW. Its abrasion rate in simulated marine environments is less than 50% of that of Q460C steel. All properties meet the technical requirements for 1000 MPa-class marine mining pipelines. Attached Figure Description

[0039] Figure 1 This is a metallographic image of the microstructure of Embodiment 1 of the present invention.

[0040] Figure 2 This is a characteristic diagram of the nanoscale precipitated phase in Example 1 of the present invention. Detailed Implementation

[0041] The present invention will be further illustrated below through examples.

[0042] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, continuous casting, heating, rolling, accelerated cooling, and slow cooling.

[0043] Heating: The slab adopts a multi-stage stepped temperature heating method, in which the high-temperature heating section has a temperature of 1200~1250℃ and the soaking section has a temperature of 1180~1220℃.

[0044] Rolling: Steel plate rolling is carried out in two stages: roughing and finishing. The finishing temperature of the roughing stage is ≥1050℃, and the single-pass reduction rate of the last two passes of roughing is ≥14%. The intermediate billet thickness of the finishing stage is 3.5~4.0t, where t is the thickness of the finished steel plate. The initial rolling temperature is 850~900℃, the single-pass reduction rate of the first three passes of finishing is ≥13%, and the final rolling temperature is 780~820℃.

[0045] Accelerated cooling: After the steel plate is rolled, accelerated cooling is carried out in two stages. The first stage starts at a cooling temperature of 760~800℃, with a cooling rate of 5~10℃ / s and a cooling time of 8~10s. The second stage cools at a cooling rate of 15~20℃ / s and a final cooling temperature of 360~430℃.

[0046] Slow cooling: After the steel plates have completed accelerated cooling, they are slowly cooled by stacking, with a cooling rate of ≤0.2℃ / s, until they reach room temperature.

[0047] Furthermore, the time for the high-temperature heating section plus the soaking section shall not be less than 120 minutes.

[0048] The composition of the steels in the embodiments of the present invention is shown in Table 1. The main process parameters for heating the steels in the embodiments of the present invention are shown in Table 2. The main process parameters for rolling the steels in the embodiments of the present invention are shown in Table 3. The main process parameters for cooling the steels in the embodiments of the present invention are shown in Table 4. The properties of the steels in the embodiments of the present invention are shown in Table 5. The microstructure of the steels in the embodiments of the present invention is shown in Table 6.

[0049] Table 1. Composition of steel in the embodiments of the present invention.

[0050]

[0051] Table 2. Main process parameters for heating steel in the embodiments of the present invention.

[0052]

[0053] Table 3. Main process parameters for steel rolling in the embodiments of the present invention.

[0054]

[0055] Table 4. Main process parameters for steel cooling in the embodiments of the present invention.

[0056]

[0057] Table 5 Properties of the steels in the embodiments of the present invention and the steels in the embodiments

[0058]

[0059] Table 6. Microstructure of the steel in the embodiments of the present invention and the steel in the embodiments.

[0060]

[0061] The steel plate produced using this invention has a microstructure consisting of a mixture of lower bainite, lath martensite, and nanoscale V-containing precipitates, expressed as follows by mass percentage: lower bainite 30%~50%; nanoscale precipitates 1.0%~2.0%, of which over 90% of the precipitates are ≤40nm in size. The steel plate exhibits a yield strength of 1000~1100MPa, a tensile strength of 1150~1250MPa, and an elongation ≥18%. The steel plate also has an impact energy ≥110J at -20℃, a fatigue strength ≥250MPa after 107 cycles, and a hardness ≥370HBW.

[0062] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A 1000MPa grade fatigue-resistant steel plate for marine environment ore transport pipelines, characterized in that, The composition of the steel plate, by weight percentage, is as follows: C: 0.13%~0.17%, Si: 0.31%~0.50%, Mn: 0.96%~1.20%, P≤0.015%, S≤0.001%, Cr: 0.70%~1.10%, Mo: 0.25%~0.45%, Al: 0.010%~0.050%, Nb: 0.08%~0.12%, V: 0.11%~0.20%, Ti: 0.005%~0.025%, Ni: 0.31%~0.50%. Ce: 0.01~0.06%, balance being Fe and unavoidable impurities; the microstructure of the steel plate is a mixture of lower bainite + lath martensite + nanoscale V-containing precipitates, with the following area percentages: lower bainite 30%~50%; nanoscale precipitates 1.0%~2.0%, of which more than 90% of the precipitates are ≤40nm in size; the yield strength of the steel plate is 1000~1100MPa, the tensile strength is 1150~1250MPa, and the elongation is ≥18%; the impact energy of the steel plate at -20℃ is ≥110J, 10 7 Cycle fatigue strength ≥250MPa, hardness ≥370HBW.

2. The steel plate for ore transportation pipelines with 1000MPa grade marine environment fatigue resistance according to claim 1, characterized in that, The steel plate contains Cr+Mo≥1.10%.

3. The steel plate for ore transportation pipelines with 1000MPa grade marine environment fatigue resistance according to claim 1, characterized in that, The Nb / Ce ratio in the steel plate is ≥1.

8.

4. The steel plate for ore transportation pipelines with 1000MPa grade marine environment fatigue resistance according to claim 1, characterized in that, The V / Ce ratio in the steel plate is ≥2.

2.

5. A method for producing a 1000MPa grade marine environment fatigue-resistant ore transport pipeline steel plate as described in any one of claims 1-4, comprising smelting, continuous casting, heating, rolling, accelerated cooling, and slow cooling; characterized in that: Heating: The slab adopts a multi-stage stepped temperature heating method, in which the high-temperature heating section has a temperature of 1200~1250℃ and the soaking section has a temperature of 1180~1220℃. Rolling: Steel plate rolling is carried out in two stages: roughing and finishing. The finishing temperature of the roughing stage is ≥1050℃, and the single-pass reduction rate of the last two passes of roughing is ≥14%. The intermediate billet thickness of the finishing stage is 3.5~4.0t, where t is the thickness of the finished steel plate. The initial rolling temperature is 850~900℃, the single-pass reduction rate of the first three passes of finishing is ≥13%, and the final rolling temperature is 780~820℃. Accelerated cooling: After the steel plate is rolled, accelerated cooling is carried out in two stages. The first stage starts at a cooling temperature of 760~800℃, with a cooling rate of 5~10℃ / s and a cooling time of 8~10s. The second stage cools at a cooling rate of 15~20℃ / s and a final cooling temperature of 360~430℃. Slow cooling: After the steel plates have completed accelerated cooling, they are slowly cooled by stacking, with a cooling rate of ≤0.2℃ / s, until they reach room temperature.

6. The method for producing a 1000MPa grade marine environment fatigue-resistant ore transport pipeline steel plate according to claim 5, characterized in that: The time for the high-temperature heating section plus the soaking section shall not be less than 120 minutes.

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