A thick steel plate with high fatigue strength in the core (490 MPa) and its manufacturing method.

Abstract

This invention provides a thick steel plate with a core fatigue strength of 490 MPa and its manufacturing method. The chemical composition of the thick steel plate, by mass percentage, comprises: C: 0.045–0.076%, Si: 0.19–0.31%, Mn: 0.95–1.13%, P: ≤0.008%, S: ≤0.002%, Als: 0.010–0.040%, Nb: 0.014–0.038%, V: 0.025–0.041%, Ti: 0.011–0.022%, Ni: 1.35–1.55%, Ce: 0.020–0.040%, Fe, and other unavoidable impurities. This thick steel plate exhibits excellent core fatigue performance and can be used in construction, engineering machinery, marine engineering, and other applications requiring specific core fatigue performance of steel plates for support components and parts.

Description

Technical Field

[0001] This invention belongs to the field of materials technology, and specifically relates to a thick steel plate with excellent core fatigue strength of 490MPa and its manufacturing method. Background Technology

[0002] With the development of my country's economy and the continuous progress of equipment design and manufacturing capabilities, the demand for high-strength steel for large components is increasing, and the main structural materials are constantly developing towards higher strength, higher toughness, and greater thickness. Thick steel plates are important structural materials for large structures, equipment, and facilities such as high-rise buildings, marine development, crude oil tanks, oil and gas pipelines, ships, and warships.

[0003] Currently, thick steel plates in China are typically produced by rolling large-thickness cast slabs. However, due to the solidification characteristics of most continuously cast slabs and limitations of existing smelting equipment and processes, the slab cross-sections suffer from uneven chemical composition distribution, internal segregation, porosity, shrinkage cavities, and other serious defects, all of which significantly impact the steel plate's strength, toughness, and fatigue performance. In particular, central segregation in the cast slab is "inherited" during the rolling process, causing abnormal microstructure and non-compliance in flaw detection at the center of the plate. This leads to premature failure in subsequent processing steps or during use due to the presence of a brittle zone in the core, affecting the safety of steel structural components. Furthermore, current domestic inspection standards often represent the performance at one-quarter of the plate's thickness, offering little constraint on the core's performance. Therefore, for thick steel plates, especially in harsh service environments with high requirements for toughness and fatigue performance, improving the comprehensive mechanical properties of the central region is crucial.

[0004] In the prior art, Chinese patent CN107641760B discloses a hot-rolled automotive structural steel sheet with good fatigue performance at 460MPa and its manufacturing method. Its chemical element composition and weight percentages are: C: 0.03–0.06%, Mn: 1.0–1.2%, Nb: 0.025–0.035%, Ti: 0.025–0.035%, Si: <0.10%, S: ≤0.005%, P: ≤0.015%, N: ≤40ppm, Als: 0.025–0.050%, with the balance being iron and unavoidable impurities. A short-process Nb-Ti microalloying technology is used to fix the S and N in the steel (S≤0.005%, N≤40ppm) with trace amounts of Ti, reducing MnS inclusions in the steel, fully utilizing the grain-refining effect of Nb and Ti, obtaining fine F+P, achieving higher strength while improving the fatigue performance of the steel. The invention is produced using rolling, laminar flow, and coiling processes, with a thickness of 1.0–3.0 mm, a yield strength of 460–560 MPa, and a tensile strength of 500–640 MPa.

[0005] Chinese patent CN109161795A discloses a high-strength automotive beam steel with good fatigue and formability, and its manufacturing method. The chemical composition and weight percentage of the steel are as follows: C: 0.04-0.07%, Si: 0.05-0.15%, Mn: 1.3-1.6%, P: ≤0.013%, S: ≤0.004%, Nb: 0.02-0.04%, Ti: ≤0.002%, N: ≤0.004%, Alt: 0.010-0.030%, O: ≤0.002%, and Ti / O < 2, with the remainder being Fe and unavoidable impurities. The automotive beam steel produced by this invention, with a thickness of 2.0–7.0 mm, achieves mechanical properties of ReL ≥ 480 MPa, Rm ≥ 600 MPa, and A ≥ 20%, resulting in a 17% weight reduction for parts. After road testing on a combined 8000 km stretch of road, the crossbeams remained intact. This high-strength automotive beam steel not only possesses excellent fatigue performance but also superior formability. The steel grade has a thickness of 2.0–7.0 mm and mechanical properties of ReL ≥ 480 MPa, Rm ≥ 600 MPa, and A ≥ 20%.

[0006] Chinese patent CN113174535A discloses a thick, quenched and tempered FO460 marine steel plate and its manufacturing method. Its chemical element content is: C: 0.06–0.10%; Si: 0.05–0.14%; Mn: 1.40–1.80%; S: ≤0.002%; P: ≤0.008%; Als: 0.015%–0.045%; N: 0.003%–0.015%; Nb: 0.01–0.04%; Cu: 0.16–0.35%; Ni: 0.30–0.60%; Cr: 0.15–0.30%; Ti: 0.008–0.014%; the balance being Fe and unavoidable impurities. This material can meet the technical requirements of marine steel plates for harsh and demanding marine environments. The invention uses steel plates with a thickness of 60-100mm, a yield strength ≥460MPa, and a tensile strength ≥570MPa.

[0007] Chinese patent CN112981235A discloses a quenched and tempered steel plate for building structures with a yield strength of 420 MPa and its production method. The composition is designed based on an Fe-Mn-C system with Nb and Ti micro-alloying treatment. The chemical composition percentages of the steel plate material are as follows: C: 0.13–0.18%, Si: 0.20–0.50%, Mn: 1.40–1.70%, P: ≤0.015%, S: ≤0.005%, Cr: ≤0.30%, Mo: ≤0.30%, Ni: ≤0.30%, Cu: ≤0.30%, Al: 0.020–0.050%, V: ≤0.015%, Nb: 0.025–0.050%, Ti: 0.010–0.020%, N: ≤0.006%, with the remainder being Fe and unavoidable impurity elements. The production process is as follows: The raw materials are sequentially smelted in a converter, refined in a ladle, and refined in an RH furnace to obtain high-purity molten steel. The molten steel is then poured into 370mm–450mm steel billets. Following this, a suitable billet heating, rolling, online direct quenching, and tempering heat treatment technology is employed to obtain a steel plate material with a yield strength of 420MPa, suitable for large steel structure construction projects such as high-rise buildings, large-span stadiums, airports, convention centers, and industrial plants. This Fangming steel grade has a thickness of 50–100mm, a yield strength ReL of 410–540MPa, and a tensile strength Rm of 530–680MPa.

[0008] Chinese patents CN107641760B and CN109161795A disclose automotive steel with good fatigue properties, but the thickness specifications are only 1-7mm, and the technical methods are not applicable to the manufacture of thick steel plates. Chinese patents CN113174535A and CN112981235A disclose two methods for manufacturing thick steel plates, but do not mention the fatigue properties of the steel plates. Summary of the Invention

[0009] The purpose of this invention is to provide a thick steel plate with excellent core fatigue strength of 490MPa and its manufacturing method. The steel plate of this invention has a thickness of 60-100mm, a yield strength ≥490MPa, a tensile strength ≥600MPa, and a core fatigue strength ≥340MPa, exhibiting excellent core fatigue performance. The strength, toughness, and fatigue performance of the steel plate are further improved, especially the comprehensive mechanical properties of the core area are excellent. This invention can solve the problems of uneven cross-sectional chemical composition distribution, internal segregation, and premature failure caused by brittle zones in the core that occur in high-strength steel for large components. It is particularly suitable for applications with high requirements for toughness and fatigue performance.

[0010] To achieve the above objectives, this invention proposes a thick steel plate with a core fatigue strength of 490 MPa. The chemical composition of the thick steel plate, by mass fraction, comprises:

[0011] C: 0.045–0.076%, Si: 0.19–0.31%, Mn: 0.95–1.13%, P: ≤0.008%, S: ≤0.002%, Als: 0.010–0.040%, Nb: 0.014–0.038%, V: 0.025–0.041%, Ti: 0.011–0.022%, Ni: 1.35–1.55%, Ce: 0.020–0.040%, Fe and other unavoidable impurities.

[0012] The thick steel plate may contain one or both of the following: B: 0.0005-0.0009%, Mo: 0.15-0.25%.

[0013] Furthermore, in the thick steel plate with excellent core fatigue performance described in this invention, by making its microstructure quasi-polygonal ferrite (QF) + lath bainite (BF) + pearlite (P), wherein the proportion of QF phase is 30-60%, the proportion of BF phase is 40-70%, and the proportion of P phase is 0.1-3%, the steel can be further guaranteed to have good strength and toughness properties.

[0014] Furthermore, in the thick steel plate with excellent core fatigue performance described in this invention, by making its average grain size 8-12μm, the strength, toughness and fatigue performance of the steel plate can be further effectively improved.

[0015] Furthermore, in the thick steel plate with excellent core fatigue performance described in this invention, the oxide inclusions are mainly Ce2O3+Al2O3, Ce2O3, Al2O3, and composite inclusions with each of them as the core, wherein the proportion of Ce2O3+Al2O3 and composite inclusions with them as the core is more than 90%, the proportion of Ce2O3 and composite inclusions with them as the core is 1-10%, and the proportion of Al2O3 and composite inclusions with them as the core is less than 1%.

[0016] In the above technical solution, controlling the type of inclusions has the effect of refining grains and promoting bainitic phase transformation.

[0017] Furthermore, in the thick steel plate with excellent core fatigue performance described in this invention, the inclusion density is 100-500 inclusions / mm². 2 The proportion of inclusions with a size of 0.2-2μm is above 95%, the proportion of inclusions with a size of >2-5μm is below 5%, the proportion of inclusions with a size of >5-10μm is below 0.01%, and there are no inclusions larger than 10μm.

[0018] In the above technical solution, the micro-control of inclusion size has the effect of refining grains, promoting bainitic phase transformation, and improving the strength, toughness and fatigue strength of steel plates.

[0019] This invention also provides a method for manufacturing a thick steel plate with high fatigue strength in the core, rated at 490 MPa, wherein the preferred steps are:

[0020] 1) Desulfurize the molten iron and control the sulfur content in the molten iron to ≤ 0.002%;

[0021] 2) Vacuum treatment time ≥ 21 min; continuous casting speed 0.5-1.0 m / min; two-stage electromagnetic stirring with current parameters of 420 A and 455 A respectively; dynamic light pressure solidity 0.35~0.70, and reduction amount 6-10 mm;

[0022] 3) Conventional continuous casting to form billets and heating the billets, controlling the heating temperature between 1201 and 1245℃, with a tapping temperature not lower than 1180℃:

[0023] 4) A two-stage rolling process is adopted. The initial rolling temperature of the first stage is not lower than 1063℃, with a single-pass reduction of ≥20mm and a reduction of ≥40mm in the last two passes. The initial rolling temperature of the second stage is not higher than 943℃, with a reduction rate of greater than 15% in the first two passes and a reduction rate of 8-10% in the remaining rolling passes. The final rolling temperature is between 821-843℃. After final rolling, rapid cooling is performed at a rate of 0.5-5℃ / s, and the re-heating temperature is controlled not to exceed 430℃.

[0024] 5) Perform industrial furnace tempering heat treatment at a temperature of 611–631℃ for a time of 1.5 min (product thickness / mm). After reaching the set temperature, maintain the temperature for at least 0.9 min (product thickness / mm). After tempering, air cool to room temperature.

[0025] Invention Effects

[0026] The steel plate of this invention has a thickness of 60-100mm, a yield strength ≥490MPa, a tensile strength ≥600MPa, and a fatigue strength ≥340MPa at the center of the plate, exhibiting excellent core fatigue performance. It can be used in support components and parts for construction, engineering machinery, marine engineering, and other applications where specific core fatigue performance requirements exist. This invention has advantages such as simple manufacturing processes and can be implemented in various metallurgical enterprises. Detailed Implementation

[0027] The present invention will be described in detail below with reference to specific embodiments and examples. However, those skilled in the art should understand that these specific embodiments and examples are for illustrating the present invention and not for limiting the present invention.

[0028] Throughout this specification, unless otherwise specified, the terminology used herein should be understood as having the meaning commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the event of any conflict, this specification shall prevail.

[0029] The present invention provides the following technical solution to achieve the above-mentioned effects:

[0030] According to a typical embodiment of the present invention, a thick steel plate with a core fatigue strength of 490 MPa and a method for manufacturing the same are provided. The chemical composition of the thick steel plate, by mass percentage, comprises: C: 0.045–0.076%, Si: 0.19–0.31%, Mn: 0.95–1.13%, P: ≤0.008%, S: ≤0.002%, Als: 0.010–0.040%, Nb: 0.014–0.038%, V: 0.025–0.041%, Ti: 0.011–0.022%, Ni: 1.35–1.55%, Ce: 0.020–0.040%, Fe, and other unavoidable impurities.

[0031] The thick steel plate may contain one or both of the following: B: 0.0005-0.0009%, Mo: 0.15-0.25%.

[0032] The control principle of each chemical component in this invention is as follows:

[0033] C and Mn are highly effective elements for improving the strength of steel. On the one hand, increasing the carbon content increases the tensile strength and yield strength of steel, but decreases the elongation and impact toughness. Furthermore, hardening occurs in the heat-affected zone of the weld, leading to cold cracking. With increasing Mn content, the strength of the steel increases significantly, while the impact transformation temperature remains almost unchanged. Mn also expands the austenite region; increasing Mn content improves austenite stability, reduces the critical cooling rate, strengthens ferrite, significantly improves hardenability, and slows down the decomposition transformation rate during tempering after quenching, thus improving the stability of the tempered structure. On the other hand, C and Mn are elements that easily segregate at the center of the billet, causing central segregation. Therefore, this invention controls C and Mn at relatively low levels (C: 0.045–0.076%, Mn: 0.95–1.13%), using other alloying elements to balance the strength and toughness of the steel plate and mitigate the adverse effects of central segregation on core properties.

[0034] Si (silicon) can increase the hardness and strength of the solid solution in steel. It not only increases the hardenability of steel but also enhances the tempering resistance of quenched steel, allowing it to be tempered at higher temperatures, thereby improving its toughness and resistance to delayed fracture. Si can significantly improve the elastic limit, yield strength, and yield ratio of steel. Excessive Si content can worsen the thermal conductivity of steel, making ingots and billets more prone to cracking or other defects. The Si content of the steel in this invention is designed to be 0.19–0.31%.

[0035] P and S are impurity elements in steel and are also prone to segregation. They can form severe segregation and inclusions in localized areas of the steel, reducing its plasticity and toughness. The steel of this invention strictly controls the sulfur and phosphorus content levels in terms of metallurgical quality, i.e., P ≤ 0.008% and S ≤ 0.002%.

[0036] Al is the main deoxidizing element in steel. Furthermore, Al has a high melting point. During production, Al in steel can combine with N to form AlN, and AlN can hinder the growth of austenite at high temperatures, thus refining the grain size. The Al content of the steel in this invention is controlled at 0.010–0.040%.

[0037] Nb and Ti are two strong carbide and nitride forming elements, exhibiting a strong affinity for nitrogen and carbon, and forming extremely stable carbonitrides. The dispersed distribution of Nb carbonitride second-phase particles along the austenite grain boundaries significantly increases the coarsening temperature of the original austenite grains. Within the austenite recrystallization temperature range during rolling, Nb carbonitride precipitates can act as nucleation sites for austenite grains. In the non-recrystallization temperature range, the dispersed Nb carbonitride precipitates effectively pin the austenite grain boundaries, preventing further austenite grain growth, thereby refining the ferrite grains and improving strength and impact toughness. Ti nitrides effectively pin austenite grain boundaries, helping to control austenite grain growth and significantly improving the low-temperature toughness of the weld heat-affected zone. Therefore, through the grain refinement and precipitation strengthening effects of Nb and Ti microalloying elements, steel plates can achieve excellent strength and toughness. The steel of this invention has an Nb content of 0.014–0.038% and a Ti content of 0.011–0.022%.

[0038] Volatile metal (V) is a potent carbide-forming element that can enhance the strength of steel through grain refinement, precipitation strengthening, and solid solution strengthening. Furthermore, in steel, when the mass fraction of V is below 0.1%, the ductile-brittle transition temperature decreases with increasing V content. Conversely, when the mass fraction of V exceeds 0.1%, increasing the V content leads to an increase in the ductile-brittle transition temperature. In steels containing Si and Mn, adding a small amount of V can significantly mitigate the effects of these two elements on grain growth and the increase in the ductile-brittle transition temperature. When V is added in combination with Nb, it can improve both the strength and toughness of the steel. The V content in this invention is 0.025-0.041%.

[0039] Ni strengthens the ferrite matrix in steel, suppresses coarse proeutectoid ferrite, significantly improves the toughness of steel, lowers the ductile-brittle transition temperature, and enhances the low-temperature impact toughness of steel. In this invention, the Ni content is designed to be 1.35–1.55%.

[0040] Ce is a rare earth element with a strong affinity for oxygen and sulfur, exhibiting purifying and significant modifying effects in steel. When dissolved in steel, it can accumulate at grain boundaries through diffusion, reducing the segregation of inclusion elements at these boundaries. This strengthens the grain boundaries and improves grain-bound properties such as low-temperature brittleness and toughness. In this invention, the Ce addition amount is 0.020-0.040%.

[0041] The main role of molybdenum (Mo) in steel is solid solution strengthening. A small amount of Mo can form refractory carbides, hindering austenite grain growth during heating, refining the microstructure, and improving strength, hardness, and wear resistance. Mo can improve hardenability, reduce or eliminate temper brittleness caused by other alloying elements, thus greatly benefiting the toughness of steel, improving tempering stability, and effectively eliminating or reducing residual stress in steel. However, excessive Mo content easily leads to coarse martensite during rapid cooling and welding cooling, reducing the low-temperature toughness of the base material and deteriorating weldability. Therefore, this invention preferably controls the Mo content to 0.15–0.25%.

[0042] Bo is an element that strongly improves hardenability. The addition of Bo can effectively inhibit the nucleation and growth of proeutectoid ferrite. Due to the non-equilibrium segregation of Bo at the austenite grain boundaries, it strongly inhibits the γ-α phase transformation and promotes the formation of fine low-carbon martensite during quenching, thereby improving the yield strength and tensile strength of steel. The preferred Bo content in this invention is 0.0005 to 0.0009%.

[0043] According to another typical embodiment of the present invention, a method for manufacturing a thick steel plate with a core fatigue strength of 490 MPa is provided, the steps of which preferably include:

[0044] 1) Desulfurize the molten iron and control the sulfur content in the molten iron to ≤ 0.002%;

[0045] 2) Vacuum treatment time ≥ 21 min; continuous casting speed 0.5-1.0 m / min; two-stage electromagnetic stirring with current parameters of 420 A and 455 A respectively; dynamic light pressure solidity 0.35~0.70, and reduction amount 6-10 mm;

[0046] 3) Conventional continuous casting to form billets and heating the billets, controlling the heating temperature between 1201 and 1245℃, with a tapping temperature not lower than 1180℃:

[0047] 4) A two-stage rolling process is adopted. The initial rolling temperature of the first stage is not lower than 1063℃, with a single-pass reduction of ≥20mm and a reduction of ≥40mm in the last two passes. The initial rolling temperature of the second stage is not higher than 943℃, with a reduction rate of greater than 15% in the first two passes and a reduction rate of 8-10% in the remaining rolling passes. The final rolling temperature is between 821-843℃. After final rolling, rapid cooling is performed at a rate of 0.5-5℃ / s, and the re-heating temperature is controlled not to exceed 430℃.

[0048] 5) Perform industrial furnace tempering heat treatment at a temperature of 611–631℃ for a time of 1.5 min (product thickness / mm). After reaching the set temperature, maintain the temperature for at least 0.9 min (product thickness / mm). After tempering, air cool to room temperature.

[0049] The key design points and rationale for the manufacturing method of this invention are as follows:

[0050] By controlling the continuous casting speed, employing a two-stage electromagnetic stirring system, and controlling the dynamic light reduction range and total reduction within the range of the present invention, defects such as center segregation of the billet can be improved to the greatest extent.

[0051] By controlling the billet heating temperature and furnace exit temperature within the range of the present invention, it can be further ensured that the steel of the present invention forms sufficient austenitization.

[0052] This invention employs a two-stage rolling process, varying the reduction per pass rather than a simple large reduction, few-pass rolling process. This allows for the superimposed refinement of recrystallized grains and ferrite nucleation grains in both stages. Furthermore, by controlling the initial rolling temperature of the first stage to be no lower than 1063°C, combined with the single-pass reduction, the rolling pressure can be effectively transmitted to the center of the billet, fully refining the austenite grains. By controlling the initial rolling temperature of the second stage to be no higher than 943°C, limiting the reduction rate of the first two passes to greater than 15%, and the reduction rate of the remaining rolling passes to 8-10%, with the final rolling temperature between 821-843°C, the superimposed refinement of recrystallized grains and ferrite nucleation grains can be further achieved, further improving the toughness of the steel plate and indirectly providing sufficient time to reduce center segregation and center porosity.

[0053] After rolling, the steel plate is cooled. By controlling the cooling rate and keeping the reheat temperature below 430°C, the steel plate can obtain a quasi-polygonal ferrite + lath bainite + pearlite microstructure and achieve the required proportions to obtain excellent basic and fatigue properties.

[0054] The tempering temperature should be controlled between 611 and 631°C. Too low a tempering temperature will result in insufficient precipitation of some elements during tempering, leading to insufficient strength and poor toughness in the steel plate; too high a temperature will cause a decrease in the strength of the steel. Furthermore, the furnace temperature will decrease when the steel plate enters the industrial furnace for tempering. Therefore, it is essential to control the holding time after tempering to a sufficient temperature, not less than (product thickness / mm × 0.9) min, to ensure sufficient precipitation and diffusion of elements during tempering and adequate elimination of internal stress, thereby achieving excellent overall performance.

[0055] By employing the chemical composition and manufacturing method of this invention for smelting, rolling, cooling, and tempering process parameters, steel plates that meet the requirements of this invention can be manufactured.

[0056] The present invention will be further described in detail below with reference to embodiments, comparative examples, and experimental data.

[0057] Example

[0058] Thick steel plates were obtained by using the different steel compositions shown in Table 1 and the process shown in Table 2.

[0059] Table 1. Chemical components and mass percentage content of each embodiment and comparative example of the present invention.

[0060]

[0061]

[0062] The specific process parameters for Examples 1 to 8 and the comparative examples are shown in Table 2:

[0063] Table 2 Main process parameters of each embodiment and comparative example of the present invention

[0064]

[0065] The comprehensive performance test results of the steel plates in Examples 1 to 8 are shown in the table below. Among them, the fatigue strength was measured according to GB / T3075 "Metallic Materials Fatigue Test Axial Force Control Method", and the sample was taken from the center part; the yield strength and tensile strength were measured according to GB / T228.1-2010 standard.

[0066] Table 3 Performance test results of various embodiments of the present invention

[0067] Example Specifications / mm Yield strength / MPa Tensile strength / MPa Fatigue strength / MPa 1 60 533 650 387 2 95 507 608 361 3 81 523 617 367 4 76 521 615 360 5 85 519 623 374 6 100 495 610 356 7 71 517 620 369 8 63 528 644 376 Comparative Example 1 70 408 553 241 Comparative Example 2 100 455 560 208 Comparative Example 3 80 500 605 210

[0068] As shown in the table, the fatigue strength of the embodiments of the present invention is all above 340 MPa, which is significantly higher than that of the comparative example, and has good cardiac fatigue performance.

[0069] The steel plate of this invention is a 60-100mm thick steel plate with a yield strength ≥490MPa, tensile strength ≥600MPa, and fatigue strength at the center of the plate thickness ≥340MPa. It can be used in construction, engineering machinery, marine engineering, and other applications requiring specific fatigue performance at the core of the steel plate for support components and parts. This invention has advantages such as simple manufacturing processes and can be implemented in various metallurgical enterprises.

[0070] Furthermore, the combination of technical features in this case is not limited to the combinations described in the specific implementation methods and embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other. All modifications that those skilled in the art can directly derive or conceive from the content disclosed in this invention should fall within the protection scope of this invention.

Claims

1. A thick steel plate with a core fatigue strength of 490 MPa, characterized in that, The chemical composition of the thick steel plate, by mass fraction, comprises: C: 0.045–0.076%, Si: 0.19–0.31%, Mn: 0.95–1.13%, P: ≤0.008%, S: ≤0.002%, Als: 0.010–0.040%, Nb: 0.014–0.038%, V: 0.025–0.041%, Ti: 0.011–0.022%, Ni: 1.35–1.55%, Ce: 0.020–0.040%, Fe and other unavoidable impurities; The microstructure of the steel plate consists of 30-60% quasi-polygonal ferrite, 40-70% lath bainite, and 0.1-3% pearlite, with an average grain size of 8-12 μm. The oxide inclusions in the steel plate are mainly Ce2O3+Al2O 3、 Ce2O 3、 Al2O3 and its respective core composite inclusions, wherein Ce2O3+Al2O3 and its core composite inclusions account for more than 90%, Ce2O3 and its core composite inclusions account for 1-10%, and Al2O3 and its core composite inclusions account for less than 1%. The density of the inclusions is 100-500 inclusions / mm². 2 The proportion of inclusions with a diameter of 0.2-2µm is above 95%, the proportion of inclusions with a diameter greater than 2µm and less than 5µm is below 5%, the proportion of inclusions with a diameter greater than 5µm and less than 10µm is below 0.01%, and there are no inclusions with a diameter greater than 10µm. The thick steel plate has a yield strength ≥ 490 MPa and a tensile strength ≥ 600 MPa. The thickness of the steel plate is 60-100mm, and the fatigue strength at the center of the plate is ≥340MPa.

2. The thick steel plate as described in claim 1, characterized in that, It further contains one or both of B: 0.0005-0.0009% and Mo: 0.15-0.25%.

3. The thick steel plate as described in claim 1 or 2, characterized in that, V：0.032-0.041%。 4. A method for manufacturing a thick steel plate with high fatigue strength in the core, rated at 490 MPa, comprising: 1) Smelting and desulfurizing of molten iron according to any one of claims 1 to 3, and controlling the S content in the molten iron to be ≤0.002%; 2) Vacuum treatment time ≥ 21 min; continuous casting speed 0.5-1.0 m / min; two-stage electromagnetic stirring; dynamic light pressing solid fraction 0.35~0.70, pressing amount 6-10 mm; 3) Continuous casting to form a billet and heating the billet; 4) Two-stage rolling is adopted; 5) Perform tempering heat treatment at a temperature of 611-631℃. The time in the furnace is (product thickness / mm × 1.5) min, and the holding time after reaching the temperature is not less than (product thickness / mm × 0.9) min. After tempering, air cool to room temperature.

5. The method for manufacturing a thick steel plate as described in claim 4, characterized in that, In step 3), the heating temperature is controlled between 1201 and 1245°C, and the furnace exit temperature is not lower than 1180°C.

6. The method for manufacturing a thick steel plate as described in claim 4 or 5, characterized in that, In step 4), the initial rolling temperature in the first stage is not lower than 1063℃; the initial rolling temperature in the second stage is not higher than 943℃; and the final rolling temperature is between 821℃ and 843℃. After final rolling, the temperature is cooled at a rate of 0.5℃ to 5℃ / s, and the temperature at which the temperature of ...

7. The method for manufacturing a thick steel plate as described in claim 6, characterized in that, In step 4), during the first stage of rolling, the reduction per pass is ≥20mm, and the reduction for the last two passes is ≥40mm; during the second stage of rolling, the reduction rate for the first two passes is greater than 15%, and the reduction rate for the remaining rolling passes is controlled between 8% and 10%.

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