A fatigue-resistant and corrosion-resistant 500mpa-grade steel plate for offshore wind power and a preparation method thereof

Abstract

This invention relates to a fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate and its preparation method. The chemical composition by mass percentage is as follows: C: 0.10%~0.12%, Si: 0.20%~0.30%, Mn: 1.20%~1.40%, P≤0.010%, S≤0.005%, Nb: 0.030%~0.040%, Ti: 0.018%~0.030%, Ni: 0.28%~0.38%, Cu: 0.35%~0.55%, Al: 0.020%~0.050%, with Pcm≤0.25%, and the remainder being Fe and unavoidable impurity elements. The 500MPa grade offshore wind power steel plate of this invention is produced using TMCP + tempering process, with a thickness of 30mm to 60mm, yield strength ≥500MPa, tensile strength 600 to 760MPa, elongation after fracture ≥17%, core impact energy ≥120J at -30℃, and corrosion resistance improved by more than 50% compared to conventional 355MPa grade offshore wind power steel plates. After being welded by submerged arc welding with a heat input ≥35kJ / cm, the welded joint has a tensile strength ≥600MPa and an impact energy ≥80J at -30℃; under a stress ratio of 0.5, the welded joint exhibits a tensile strength ≥600MPa and an impact energy ≥80J at -30℃. 7 With a cycle axial loading fatigue limit ≥450MPa, it combines high strength and toughness, fatigue resistance, corrosion resistance and easy welding characteristics, which can meet the needs of safe service and high strength and lightweight development of marine wind power facilities.

Description

Technical Field

[0001] This invention belongs to the field of low-alloy high-strength steel production technology, specifically relating to a fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate and its preparation method. Background Technology

[0002] Against the backdrop of global energy transition and climate change response, the development of wind power equipment towards larger scale, intelligence, and green technology is progressing rapidly. Onshore turbines are expanding from 2-4MW to 5-6MW, while offshore turbines are increasing from 6-8MW to 10-14MW, thus increasing the load on the towers. Especially in the face of harsh alternating load environments such as waves, tides, and storms at sea, higher requirements are placed on the low-temperature fracture toughness, fatigue resistance, and corrosion resistance of the steel plates for service safety. Currently, although 355MPa grade wind turbine steel plates with a thickness of less than 60mm are widely used in wind power equipment, cracking in the welded areas (especially at the circumferential welds) due to insufficient strength frequently leads to tower collapses. Therefore, to ensure the safe and reliable operation of wind power facilities, the development of wind turbine steel plates towards higher strength levels has become an irreversible trend. The application of high-strength steel can reduce energy consumption in the manufacturing process, while also allowing the tower diameter to meet transportation restrictions, reducing transportation costs, and achieving green development across the entire industry chain. Improving the strength, toughness, fatigue resistance, and weldability of steel plates, while also ensuring excellent corrosion resistance and other comprehensive performance indicators, is an important way to meet the needs of large-scale, green, and economical development of wind power and the safety performance of towers during service.

[0003] Current reports on wind power steel plates mostly focus on the 345-460 MPa range, with production processes involving TMCP (+tempering), normalizing, and normalized rolling. For example, patent application CN114635074A discloses a method for producing 80-100 mm thick wind power steel plates using the TMCP process, with a maximum yield strength of only 395 MPa; patent application CN112662933A discloses a method for preparing low-temperature impact-resistant and tough wind power steel using a normalized rolling process, achieving a strength of 420 MPa; patent CN113969372B discloses a low-carbon fatigue-resistant wind power steel plate and its preparation method, with a yield strength ≥465 MPa, and according to its implementation method, the highest yield strength is 475 MPa. It is evident that none of the above technologies can meet the demand of the offshore wind power facility manufacturing industry for 500 MPa grade steel plates.

[0004] Patent application CN116219320A discloses a lightweight pile steel for marine environments and its production method. The composition includes C: 0.03%–0.06%, Si: 0.15%–0.3%, Mn: 0.7%–1%, V: 0.04%–0.06%, N: 0.0012%–0.016%, Cu: 0.3%–0.5%, Ni: 0.8%–1.2%, Mo: 0.2%–0.4%, Ta: 0.01%–0.02%, Re: 0.01%–0.02%, Cr: 1%–1.5%, REM: 0.03%–0.05%, Sn: 0.05%–0.1%, Ti: 0.001%–0.005wt%, P: 0.01%–0.02%, S≤0.005%, and a strength of 500MPa. Its composition system is relatively complex, and it contains more precious metals such as Ni, Cr, and Mo, which increases the cost and deteriorates the welding performance. At the same time, it fails to reflect the fatigue performance of the steel plate.

[0005] Patent application CN115896612A discloses a low-carbon equivalent weathering steel with a yield strength of 500MPa and its production method. The steel has the following composition: C: 0.09%~0.12%, Si: 0.30%~0.40%, Mn: 1.20%~1.40%, P≤0.015%, S≤0.005%, Cr: 0.35%~0.50%, Cu: 0.18%~0.25%, Ni: 0.15%~0.30%, and Ti: 0.055%~0.075%. The high Ti content makes it easy to form large TiN particles, which cannot achieve the effect of refining the grains. Although it has certain resistance to atmospheric corrosion, it cannot meet the service safety requirements of the strong corrosive environment of seawater. Summary of the Invention

[0006] To address the shortcomings and defects of existing technologies, this invention aims to provide a 500MPa grade fatigue-resistant and corrosion-resistant steel plate for marine wind power and its preparation method. The 500MPa grade marine wind power steel plate of this invention is produced using a TMCP+tempering process, possessing characteristics such as high strength and toughness, fatigue resistance, corrosion resistance, and easy welding, which can meet the requirements for safe operation and high-strength, lightweight development of marine wind power facilities.

[0007] To achieve the above objectives, the first aspect of the present invention provides a compositional design for a 500MPa grade fatigue-resistant and corrosion-resistant steel plate for marine wind power, employing the following technical solution:

[0008] A fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate, wherein the chemical composition of the steel plate by mass percentage is: C: 0.10%~0.12%, Si: 0.20%~0.30%, Mn: 1.20%~1.40%, P≤0.010%, S≤0.005%, Nb: 0.030%~0.040%, Ti: 0.018%~0.030%, Ni: 0.28%~0.38%, Cu: 0.35%~0.55%, Al: 0.020%~0.050%, and Pcm≤0.25%, with the remainder being Fe and unavoidable impurity elements.

[0009] Where: Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B, the element symbols in the formula represent the mass percentage content of the corresponding element in the steel plate.

[0010] In the aforementioned 500MPa grade fatigue-resistant and corrosion-resistant marine wind power steel plate, as a preferred embodiment, the unavoidable impurity elements, by mass percentage, are: H≤0.0002%, O≤0.003%, N≤0.0040%, As≤0.007%, Sb≤0.010%, Sn≤0.015%, and Pb≤0.010%.

[0011] In the aforementioned fatigue-resistant and corrosion-resistant 500MPa grade offshore wind power steel plate, as a preferred embodiment, the steel plate has a thickness of 30mm to 60mm, a yield strength ≥500MPa, a tensile strength of 600 to 760MPa, an elongation after fracture ≥17%, and a core impact energy ≥120J at -30℃. Its corrosion resistance is improved by more than 50% compared to conventional 355MPa grade offshore wind power steel plates. After submerged arc welding with a heat input ≥35kJ / cm, the welded joint has a tensile strength ≥600MPa and an impact energy ≥80J at -30℃. Under a stress ratio (the ratio of minimum loaded stress to maximum loaded stress) of 0.5, the welded joint exhibits a tensile strength ≥600MPa and an impact energy ≥80J at -30℃. 7 The fatigue limit under axial loading for each cycle is ≥450MPa.

[0012] The main alloys in the fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate play the following roles:

[0013] Ni is the element that most significantly improves the low-temperature toughness of steel plates. Adding an appropriate amount of Ni can reduce the stacking fault energy of crystals, which is beneficial for dislocation slip movement and improves impact toughness. Simultaneously, Ni can promote the formation of a dense protective rust layer on the steel plate surface, improving its corrosion resistance. However, excessively high Ni content is detrimental to weldability. Therefore, in this invention, the Ni content is controlled between 0.28% and 0.38%.

[0014] Cu can improve the corrosion resistance and strength of steel, as well as its weldability and machinability. However, excessive Cu content can increase the tendency of steel plates to become hot brittle. Therefore, the Cu content in this invention is controlled at 0.35% to 0.55%.

[0015] Nitrogen (Nb) can effectively refine grains and also play a role in precipitation strengthening; however, due to limitations imposed by carbon (C) and the influence of heating temperature, excessively high Nb content cannot achieve sufficient solid solution. Therefore, in this invention, the Nb content is controlled at 0.030%–0.040%.

[0016] Ti can also refine grains and strengthen through precipitation, significantly improving the low-temperature impact toughness of steel plates. Adding a small amount of titanium can fix nitrogen in the steel and promote the solid solution of niobium microalloying elements, but if the content is too high, large TiN particles are easily formed, thus losing the grain-refining effect. Therefore, this invention controls the Ti content to 0.018%–0.030%.

[0017] Al can fix free nitrogen in steel, improving the low-temperature toughness of steel plates and welded hard zones. Furthermore, the dispersed precipitation of AlN inhibits austenite grain growth during heating, uniformly refines the austenite grain size, and improves impact toughness. However, excessive Al content leads to an increase in the number and size of inclusions in the steel, resulting in a decrease in the internal quality of the steel plate and affecting its hot working, weldability, and machinability. Therefore, this invention controls the Al content to be between 0.020% and 0.050%.

[0018] Pcm: Controlling the cold cracking sensitivity coefficient is beneficial to ensuring the weldability of the product. This invention controls Pcm to be ≤0.25%.

[0019] Impurities in steel, such as sulfur (S) and phosphorus (P), can increase segregation in continuously cast billets, leading to poor flaw detection after welding and deteriorating the fatigue and corrosion resistance of the steel plates. Therefore, the contents of S and P are controlled below 0.005% and 0.010%, respectively.

[0020] A second aspect of this invention provides a method for preparing the above-mentioned fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate, comprising the following steps:

[0021] 1) Hot metal pretreatment: The hot metal is desulfurized using the KR method to control the sulfur content below 0.008%.

[0022] 2) Converter smelting: The process of primary refining molten iron and scrap steel to obtain primary molten steel;

[0023] 3) Steel refining: molten steel is refined by LF refining + RH refining;

[0024] 4) Billet casting: Casting to obtain continuously cast billets;

[0025] 5) Billet heating: Cold-charged billets are loaded into the furnace; a multi-stage heating method is adopted;

[0026] 6) Controlled rolling: The billet is rolled in two stages, rough rolling and finish rolling, to obtain steel plates of specific dimensions;

[0027] 7) Controlled cooling: Cooling the rolled steel plate;

[0028] 8) Tempering: Tempering heat treatment is performed on water-cooled steel plates.

[0029] In the above-mentioned method for preparing 500MPa grade steel plates for marine wind power with fatigue resistance and corrosion resistance, as a preferred embodiment, the molten iron is desulfurized by the KR method during the pretreatment of the molten iron, so that the sulfur content of the molten iron is controlled below 0.008%, and the desulfurization temperature is 1240℃~1300℃.

[0030] In the above-mentioned method for preparing 500MPa grade fatigue-resistant and corrosion-resistant marine wind power steel plates, as a preferred embodiment, during the converter smelting, molten iron after KR desulfurization treatment, scrap steel, nickel plates, and copper plates are added to the converter; preferably, the scrap steel / (molten iron + scrap steel) ≤ 10wt%. During smelting, the final slag basicity is controlled at R = 3.0~4.0, and alloying is carried out using metallic manganese, ferroniobium, and ferrosilicon. The alloy is added when 20%~30% of the steel is tapped, and the addition is completed when the molten steel reaches 70%~80%. Preferably, aluminum manganese ferrometallurgy is added at 3~3.5kg / t to deoxidize the molten steel.

[0031] This invention achieves low C, low Mn, and low P composition control during converter smelting, aiming to improve C and Mn segregation in steel plates, reduce inclusion levels, and enhance impact toughness, fatigue strength, and corrosion resistance. During this stage, Ni and Cu elements, which improve strength and corrosion resistance, as well as Nb elements, which refine grain size, must be properly balanced to avoid incomplete dissolution and failure to achieve the desired effect during subsequent refining.

[0032] In the above-mentioned method for preparing 500MPa grade fatigue-resistant and corrosion-resistant marine wind power steel plates, as a preferred embodiment, the steel refining step is divided into LF refining and RH refining. LF refining involves bottom blowing argon stirring throughout the process, using aluminum particles, calcium carbide, and silicon carbide to adjust the slag. The top slag before leaving the station is white slag, and the holding time is ≥10min to reduce the S content. The Al composition is adjusted with aluminum wire to control the oxygen content of the molten steel. Titanium readily combines with oxygen, so in this stage, titanium wire is used to adjust the titanium content to 0.018%-0.030%. Trace amounts of titanium can fix nitrogen in the steel and promote the solid solution of niobium to refine the grains and improve the strength, toughness, and fatigue resistance. RH refining ensures a vacuum degree ≤30Pa and a pure degassing time ≥5min. After RH refining, 100-120m of nano-high calcium wire is fed per furnace. If the casting furnace is shut down, the length of nano-high calcium wire fed should be ≥140m. Calcium treatment promotes the refinement and spheroidization of inclusions, which can effectively improve the impact toughness, corrosion resistance and fatigue performance of steel. Preferably, LF refining time is ≥50min, of which argon blowing time is ≥5min. Preferably, RH refining time is ≥50min, of which soft argon blowing time is ≥14min.

[0033] In the above-mentioned method for preparing 500MPa grade fatigue-resistant and corrosion-resistant marine wind power steel plates, as a preferred embodiment, full-process protective casting is adopted in the casting of the billet. The crystallizer uses peritectic steel protective slag, and the tundish is covered with carbonized rice husks to prevent oxidation of the molten steel and remove inclusions floating on the molten steel. Preferably, the superheat of the molten steel during casting is ≤15℃, and the billet casting speed is 0.80-0.10m / min. Preferably, for steel plates with a thickness of 30mm ≤ finished product thickness ≤ 50mm, a 250mm or 300mm thick billet is selected; for steel plates with a thickness of 50mm < finished product thickness ≤ 60mm, a 300mm thick billet is selected. Preferably, the billet is slowly cooled in the pit or stacked for ≥48h.

[0034] In the above-mentioned method for preparing 500MPa grade fatigue-resistant and corrosion-resistant marine wind power steel plates, as a preferred embodiment, the billet is cold-charged into the furnace during the billet heating process, and a multi-stage heating method is adopted. The first stage heating temperature is 1050-1200℃, the second stage heating temperature is 1180-1250℃, and the soaking temperature is 1160-1220℃. The furnace pressure is well controlled, and a small amount of gas and air is used in the soaking stage to keep its temperature slightly lower than that of the second stage heating, thereby inhibiting the further formation of iron oxide scale. Preferably, the soaking time is ≥50min, the furnace time is ≥10min / cm, and the tapping temperature is 1180-1200℃, which ensures that the billet is evenly and thoroughly heated while preventing excessive growth of austenite grains. Preferably, after the billet is heated, high-pressure water descaling treatment is performed.

[0035] In the above-mentioned method for preparing 500MPa grade fatigue-resistant and corrosion-resistant marine wind power steel plates, as a preferred embodiment, the rolling process in the controlled rolling step is a two-stage rolling process of roughing and finishing; preferably, the roughing is recrystallization rolling, adopting a large reduction mode, with ≤6 roughing passes, and ensuring that the reduction rate (single pass thickness reduction / entry thickness) of at least 2 passes is ≥18%; preferably, in the roughing, for steel plates with a specification of 30mm ≤ finished thickness < 40mm, the intermediate billet thickness / finished product thickness after roughing is 3.5; for steel plates with a specification of 40mm ≤ finished thickness < 50mm, the intermediate billet thickness / finished product thickness after roughing is 3.0; for steel plates with a specification of 50mm ≤ finished thickness ≤ 60mm, the intermediate billet thickness / finished product thickness after roughing is 2.4. Preferably, the finishing rolling is non-recrystallization rolling, with ≤8 finishing rolling passes; more preferably, the finishing rolling passes are 6; even more preferably, in the finishing rolling, for steel plates with a finished thickness of 30mm ≤ finished thickness < 45mm, the initial finishing rolling temperature is 840~850℃; for steel plates with a finished thickness of 45mm ≤ finished thickness ≤ 60mm, the initial finishing rolling temperature is 830~840℃; even more preferably, the deformation of the last 3 passes is carried out in the temperature range of 800~820℃.

[0036] In this invention, the intermediate billet thickness refers to the thickness of the cast billet after rough rolling. By limiting the rough rolling to recrystallization rolling with ≤6 passes and ensuring a reduction rate ≥18% for at least 2 passes, the effect of Nb is fully utilized. The high reduction rate refines the austenite grains, improving the steel plate's strength, toughness, and fatigue resistance. By limiting the finish rolling to non-recrystallization rolling with ≤8 passes, the cumulative deformation effect between passes and the forced phase transformation mechanism induced by deformation defects within the austenite grains are fully utilized. This promotes the formation of numerous deformation bands, twins, and dislocations within the austenite grains, creating conditions for subsequent phase deformation nuclei and improving the steel plate's strength, toughness, and fatigue resistance. Controlling the thickness of the intermediate billet after rough rolling allows for a reasonable allocation of the reduction amounts in rough and finish rolling, improving the overall performance of the steel plate.

[0037] In the above-mentioned method for preparing 500MPa grade fatigue-resistant and corrosion-resistant marine wind power steel plates, as a preferred embodiment, in the controlled cooling step, the initial cooling temperature is controlled at 760-780℃, and the final cooling temperature is controlled at 500-530℃; preferably, the cooling rate is 10-12℃ / s.

[0038] To ensure weldability, this invention employs a low-carbon, low-manganese composition design with low amounts of hardenability elements. Therefore, a strong water-cooling process after rolling is selected to obtain a microstructure with bainite accounting for more than 60%, which improves the strength of the steel plate while refining the microstructure and ensuring the low-temperature impact resistance and fatigue resistance of the steel plate.

[0039] In the above-mentioned method for preparing 500MPa grade fatigue-resistant and corrosion-resistant marine wind power steel plates, as a preferred embodiment, in the tempering step, for steel plates with a thickness of 30mm ≤ finished product thickness < 50mm, the heating temperature is 420±5℃ and the holding time is 15±2min; for steel plates with a thickness of 50mm ≤ finished product thickness ≤ 60mm, the heating temperature is 440±5℃ and the holding time is 20±2min.

[0040] This invention, through tempering heat treatment, can effectively eliminate the internal stress of steel plates after strong water cooling, reduce stress crack sources, eliminate the difference in corrosion points in different parts caused by uneven stress, and improve the fatigue resistance and corrosion resistance of steel plates.

[0041] In this way, a 30-60 mm thick fatigue-resistant and corrosion-resistant 500 MPa grade marine wind power steel plate is obtained. Preferably, the room temperature microstructure of the steel plate obtained by the preparation method is about 60% bainite + about 30% acicular ferrite + retained austenite.

[0042] Compared with the prior art, the advantages of the present invention are:

[0043] (1) This invention guarantees a long service life. The single-unit capacity of offshore wind turbines is generally more than twice that of onshore wind turbines, and the load on the tower is more complex. Moreover, seawater, as a highly corrosive medium, constantly threatens the service safety performance of wind turbines. This invention improves the segregation of C and Mn in the steel plate, controls the content of P and S, promotes grain refinement, and improves the characteristics of the rust layer. After controlled cooling and tempering treatment, the internal stress of the steel plate is eliminated, the generation and propagation of fatigue cracks are suppressed, and the density of corrosive inclusions and the corrosion potential difference of the structure are reduced. This makes the fatigue strength of the steel plate after welding ≥450MPa, and the corrosion resistance is improved by more than 50% compared with the conventional 355MPa grade wind power steel plate. This can straighten the "backbone" of the offshore unit and provide a strong guarantee for achieving a safe service life of 20 years.

[0044] (2) This invention achieves high welding efficiency. This invention adopts a low-carbon, low-manganese, and low-carbon equivalent composition design, obtains high-quality cast billets through high-purity steel smelting technology, and uses TMCP production process to achieve high-penetration rolling, giving full play to the effects of solid solution strengthening, precipitation strengthening and fine grain strengthening. While obtaining good strength and toughness of steel plates, it ensures excellent welding performance. The welding heat input can reach more than 35kJ / cm, effectively realizing high efficiency in the processing.

[0045] (3) This invention achieves green and lightweight design. Due to the increased strength, the wind turbine towers manufactured using the products of this invention can be more than 10% lighter than ordinary 355MPa marine wind turbine panels. At the same time, the improved corrosion resistance of the steel plates can reduce the corrosion allowance design of the steel plates when serving in highly corrosive marine environments, effectively meeting the industry development needs for increased strength in marine wind power facilities. Attached Figure Description

[0046] Figure 1 This is a metallographic diagram of the steel plate in Embodiment 1 of the present invention. Detailed Implementation

[0047] Any feature disclosed in this specification, unless specifically stated otherwise, can be replaced by other equivalent or similar features. Unless specifically stated otherwise, each feature is merely one example of a series of equivalent or similar features. The descriptions are merely to aid in understanding the invention and should not be construed as limiting the invention. The invention will now be described in further detail with reference to specific embodiments.

[0048] Example 1: The steel plate thickness is 30mm.

[0049] This invention provides a 500MPa grade fatigue-resistant and corrosion-resistant steel plate for marine wind power. After KR treatment, the S content is 0.004%. During converter smelting, Ni, Cu, and Nb elements for grain refinement must be properly balanced to avoid addition during subsequent refining. LF refining time is 50 minutes, during which titanium wire is fed to adjust the titanium content to 0.018%-0.030%. RH refining time is 55 minutes, followed by feeding 100m of nano-high-calcium wire. The smelting chemical composition and weight percentage are: C: 0.115%, Si: 0.25%, Mn: 1.25%, P: 0.010%, S: 0.002%, Nb: 0.030%, Ti: 0.018%, Ni: 0.35%, Cu: 0.50%, Al: 0.038%, with the remainder being Fe and unavoidable impurities, Pcm = 0.23%. The 250mm thick continuous casting billet was then cast at a casting speed of 0.95m / min and then slowly cooled for 48 hours after cutting.

[0050] The slowly cooled billet is then heated to a tapping temperature of 1190℃ and a furnace time of 270 minutes. Rough rolling is performed using a high reduction mode, with a total of 6 passes. The reduction rates for the 5th and 6th passes are 18.3% and 20.7%, respectively. The thickness of the intermediate billet after rough rolling is 105 mm. The finishing rolling stage begins at 850℃, with 8 passes. The rolling temperatures for the last 3 passes are 815℃, 814℃, and 810℃, respectively. After rolling, controlled cooling is performed at a rate of 12℃ / s, with an initial cooling temperature of 778℃ and a final cooling temperature of 503℃. The tempering heating temperature is 420℃, and the holding time is 15 minutes.

[0051] Example 2: The steel plate thickness is 50mm.

[0052] This invention provides a 500MPa grade fatigue-resistant and corrosion-resistant steel plate for marine wind power. After KR treatment, the S content is 0.006%. During converter smelting, Ni, Cu, and Nb elements for grain refinement must be properly balanced to avoid addition during subsequent refining. LF refining time is 58 minutes, during which titanium wire is fed to adjust the titanium content to 0.018%-0.030%. RH refining time is 55 minutes, followed by feeding 110m of nano-high-calcium wire. The smelting chemical composition and weight percentage are: C: 0.104%, Si: 0.27%, Mn: 1.31%, P: 0.009%, S: 0.002%, Nb: 0.035%, Ti: 0.020%, Ni: 0.38%, Cu: 0.42%, Al: 0.041%, with the remainder being Fe and unavoidable impurities, Pcm = 0.24%. The 250mm thick continuous casting billet was then cast at a casting speed of 0.95m / min and then slowly cooled for 48 hours after cutting.

[0053] The slowly cooled billet is heated to a tapping temperature of 1200℃ and a furnace time of 275 minutes. The rough rolling adopts a large reduction mode with a total of 5 passes. The reduction rates of the 4th and 5th passes are 19.1% and 21.2%, respectively. The thickness of the intermediate billet after rough rolling is 120 mm. The finishing rolling stage begins when the temperature reaches 837℃. The finishing rolling is carried out in 8 passes, with the rolling temperatures of the last 3 passes being 818℃, 815℃, and 813℃, respectively. After rolling, controlled cooling is carried out at a rate of 11℃ / s, with an initial cooling temperature of 770℃ and a final cooling temperature of 518℃. The tempering heating temperature is 440℃, and the holding time is 20 minutes.

[0054] Example 3: The steel plate thickness is 60mm.

[0055] This invention provides a 500MPa grade fatigue-resistant and corrosion-resistant steel plate for marine wind power. After KR treatment, the S content is 0.005%. During converter smelting, Ni, Cu, and Nb elements for grain refinement must be properly balanced to avoid addition during subsequent refining. LF refining time is 55 minutes, during which titanium wire is fed to adjust the titanium content to 0.018%-0.030%. RH refining time is 53 minutes, followed by feeding 120m of nano-high-calcium wire. The smelting chemical composition and weight percentage are: C: 0.118%, Si: 0.21%, Mn: 1.38%, P: 0.010%, S: 0.002%, Nb: 0.036%, Ti: 0.024%, Ni: 0.30%, Cu: 0.51%, Al: 0.039%, with the remainder being Fe and unavoidable impurities, Pcm = 0.24%. The 300mm thick continuous casting billet was then cast, stretched at 0.80m / min, and then slowly cooled for 48 hours after cutting.

[0056] The slowly cooled billet was heated to a tapping temperature of 1182℃ and a furnace time of 278 minutes. Rough rolling was performed using a large reduction mode, with a total of 5 passes. The reduction rates for the 4th and 5th passes were 18.9% and 21.3%, respectively. The thickness of the intermediate billet after rough rolling was 144 mm. The finishing rolling stage began at 830℃, with 6 passes. The rolling temperatures for the last 3 passes were 820℃, 815℃, and 812℃, respectively. After rolling, controlled cooling was performed at a rate of 10℃ / s, with an initial cooling temperature of 769℃ and a final cooling temperature of 521℃. The tempering heating temperature was 440℃, and the holding time was 20 minutes.

[0057] Comparative Example 1: The steel plate thickness is 60mm. The strength is relatively weak after water cooling after rolling. The specific process is as follows.

[0058] The same furnace as Example 3, with the same composition, used a 300mm thick continuously cast billet. The billet was heated to a tapping temperature of 1181℃ and held in the furnace for 277 minutes. Rough rolling was performed using a large reduction mode, with a total of 5 passes. The reduction rates for the 4th and 5th passes were 19.0% and 21.1%, respectively. After rough rolling, the intermediate billet thickness was 144mm. The finishing rolling stage began at 830℃, with 6 passes. The rolling temperatures for the last 3 finish rolling passes were 820℃, 816℃, and 812℃, respectively. After rolling, controlled cooling was performed at a rate of 9℃ / s, with an initial cooling temperature of 770℃ and a final cooling temperature of 548℃. The tempering heating temperature was 440℃, and the holding time was 20 minutes.

[0059] Comparative Example 2: The steel plate thickness is 60mm. No tempering treatment is performed after water cooling; the specific process is as follows.

[0060] Comparative Example 2 and Example 3 were from the same furnace, with the same composition, and used 300mm thick continuously cast billets. The billets were heated to a tapping temperature of 1181℃ and held in the furnace for 280 minutes. Rough rolling was performed using a high-reduction mode, with a total of 5 passes. The reduction rates for the 4th and 5th passes were 19.1% and 20.8%, respectively, resulting in an intermediate billet thickness of 144mm after rough rolling. The finishing rolling stage began at 830℃, with 6 passes. The rolling temperatures for the last 3 finish passes were 820℃, 816℃, and 814℃, respectively. After rolling, controlled cooling was performed at a rate of 10℃ / s, with an initial cooling temperature of 768℃ and a final cooling temperature of 525℃. No tempering heat treatment was subsequently performed.

[0061] Table 1 shows the mechanical properties of the steel plate base material. In Examples 1-3, the yield strength is ≥500 MPa, the tensile strength is 600–760 MPa, the elongation after fracture is ≥17%, and the core impact energy at -30℃ is ≥120 J. The room temperature microstructure of the steel plate obtained by the preparation method is approximately 60% bainite + approximately 30% acicular ferrite + retained austenite. Figure 1The image shows the metallographic structure of the obtained steel plate. In Comparative Example 1, due to the lower water-cooled strength, the amount of ferrite and pearlite formed is relatively larger, resulting in significantly lower strength than in the Example. The properties of the base material in Comparative Example 2 are similar to those in the Example.

[0062] The steel plates were welded using submerged arc welding with a welding heat input of 35.50–40.09 kJ / cm. The mechanical properties after welding are shown in Table 2. In Examples 1-3, the tensile strength of the welded joint was ≥600 MPa, and the impact energy at -30℃ was ≥80 J. The fatigue test stress ratio was 0.5, and the sinusoidal stress cycle was ≥10. 7 Secondly, the fatigue limit of the steel plate is ≥450MPa. Comparative Example 1 shows a significant decrease in strength after welding, indicating that the ferrite + pearlite-dominated microstructure deteriorates significantly after welding with a heat input of 35kJ / cm. Furthermore, due to its inherently low strength, its fatigue limit is also low. Comparative Example 2 shows similar strength to the example after welding, but its fatigue limit is very low, even lower than Comparative Example 1. This indicates that without tempering after water cooling, the internal stress of the steel plate is easily induced to form crack initiation sites, reducing fatigue strength.

[0063] The steel plates were immersed in artificial seawater (composition shown in Table 3) at a test temperature of 25±2℃ for 30 days. The corrosion rate was calculated using the weight loss method, and the results are shown in Table 4. The chemical composition of the TMCP-condition EH36 steel used for comparison is: C: 0.15%, Si: 0.33%, Mn: 1.20%, P: 0.013%, S: 0.005%, Nb: 0.018%, Ti: 0.025%, Al: 0.035%. It can be seen that the corrosion resistance of the present invention is improved by more than 50% compared with the conventional 355MPa grade marine wind power steel plate.

[0064] Table 1 Mechanical properties of the base material

[0065]

[0066] Table 2 Mechanical properties after welding

[0067]

[0068]

[0069] Table 3 Composition of artificial seawater

[0070]

[0071] Table 4 Corrosion Rate in Seawater

[0072]

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A fatigue-resistant and corrosion-resistant 500MPa grade steel plate for marine wind power, characterized in that, The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.10%~0.12%, Si: 0.20%~0.30%, Mn: 1.20%~1.40%, P≤0.010%, S≤0.005%, Nb: 0.030%~0.040%, Ti: 0.018%~0.030%, Ni: 0.28%~0.38%, Cu: 0.35%~0.55%, Al: 0.020%~0.050%, with the remainder being Fe and unavoidable impurity elements; The method for preparing the fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate includes the following steps: 1) Hot metal pretreatment: Hot metal is desulfurized using the KR method to control the sulfur content of the hot metal to below 0.008%; 2) Converter smelting: The primary refining of molten iron and scrap steel yields primary molten steel; 3) Steel refining: molten steel is refined by LF refining + RH refining; 4) Billet casting: Casting to obtain continuously cast billets; 5) Billet heating: Cold-charged billets are loaded into the furnace; a multi-stage heating method is adopted; 6) Controlled rolling: The billet is subjected to two stages of rolling, rough rolling and finish rolling, to obtain steel plates; 7) Controlled cooling: Cooling the rolled steel plate; 8) Tempering: Tempering heat treatment is performed on water-cooled steel plates; In step 8), during tempering, for steel plates with a thickness of 30mm ≤ finished product thickness < 50mm, the heating temperature is 420±5℃ and the holding time is 15±2min; for steel plates with a thickness of 50mm ≤ finished product thickness ≤ 60mm, the heating temperature is 440±5℃ and the holding time is 20±2min.

2. The fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 1, characterized in that, The mass percentage content of each component in the unavoidable impurities is as follows: H≤0.0002%, O≤0.003%, N≤0.0040%, As≤0.007%, Sb≤0.010%, Sn≤0.015%, Pb≤0.010%.

3. The fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 1, characterized in that, The steel plate has Pcm ≤ 0.25%, thickness of 30mm-60mm, yield strength ≥ 500MPa, tensile strength of 600-760MPa, elongation after fracture ≥ 17%, core impact energy at -30℃ ≥ 120J, after the steel plate is welded by submerged-arc welding with heat input ≥ 35kJ / cm, the tensile strength of the welded joint is ≥ 600MPa, the impact energy at -30℃ is ≥ 80J; the welded joint has a stress ratio of 0.5, 10 7 The number of cycles of axial loading fatigue limit is ≥ 450MPa.

4. The fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 1, characterized in that, In step 1) of the molten iron pretreatment, the desulfurization temperature is 1240℃-1300℃; In step 2) of converter smelting, the basicity of the final slag is controlled at R=3.0~4.

0. Metallic manganese, ferroniobium and ferrosilicon are used for alloying. The alloy is added when 20%~30% of the steel is tapped and the addition is completed when 70%~80% of the molten steel is tapped. Aluminum manganese ferrometallurgy is added at 3~3.5kg / t to deoxidize the molten steel.

5. The fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 1, characterized in that, In step 4), the superheat of the molten steel during casting is ≤15℃, and the casting speed is 0.80-0.10m / min.

6. The fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 1, characterized in that, In step 5), the first heating stage temperature is 1050~1200℃, the second heating stage temperature is 1180~1250℃, the soaking stage temperature is 1160~1220℃, the soaking time is ≥50min, the furnace time is ≥10min / cm, and the tapping temperature is 1180~1200℃.

7. The fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 1, characterized in that, In step 6), during controlled rolling, the roughing is recrystallization rolling, using a large reduction mode, with ≤6 roughing passes and at least 2 passes having a reduction rate ≥18%; the finishing is non-recrystallization rolling, with ≤8 finishing passes; for steel plates with a thickness of 30mm ≤ finished product < 45mm, the finishing rolling start temperature is 840~850℃; for steel plates with a thickness of 45mm ≤ finished product ≤ 60mm, the finishing rolling start temperature is 830~840℃; the final 3 deformation passes are performed within a temperature range of 800~820℃.

8. The fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 1, characterized in that, In step 7), during controlled cooling, the initial cooling temperature is controlled at 760-780℃, the final cooling temperature is controlled at 500-530℃, and the cooling rate is 10-12℃ / s.

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