Fatigue-resistant and corrosion-resistant 500 mpa-grade steel plate for marine wind power and preparation method therefor

Abstract

The present invention relates to a fatigue-resistant and corrosion-resistant 500 MPa-grade steel plate for marine wind power, the chemical composition of which is as follows, by mass percentage: 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%, Pcm being controlled to be ≤0.25%, and the remainder being Fe and unavoidable impurity elements. The 500 MPa-grade steel plate for marine wind power of the present invention is produced using a TMCP+tempering process, and has a thickness of 30 mm-60 mm, a yield strength of ≥500 MPa, a tensile strength of 600-760 MPa, an elongation after fracture of ≥17%, and a core impact energy of ≥120 J at -30℃. Corrosion resistance is improved by 50% or more compared with conventional 355 MPa-grade steel plate for marine wind power. After the steel plate is welded by submerged arc welding with a heat input of ≥35 kJ / cm, the tensile strength of the welded joint is ≥600 MPa, and the impact energy at -30℃ is ≥80 J; under conditions of a stress ratio of 0.5, the fatigue limit of a welded joint after 107 cycles of axial loading is ≥450 MPa. The steel has the characteristics of high strength, fatigue resistance, corrosion resistance and easy welding, and can meet the development requirements of safe service, high strength and light weight for marine wind power facilities.

Description

A fatigue-resistant and corrosion-resistant 500MPa-grade steel plate for marine wind power and its preparation method

[0001] Related applications

[0002] This application claims priority to Chinese patent application No. 2023114099011, entitled “A fatigue-resistant and corrosion-resistant 500MPa grade steel plate for marine wind power and its preparation method”, filed on October 27, 2023, and this application is included herein by reference. Technical Field

[0003] The present invention belongs to the technical field of low-alloy high-strength steel production, and specifically relates to a fatigue-resistant and corrosion-resistant 500MPa grade steel plate for marine wind power and a preparation method thereof. Background Art

[0004] Amidst the global energy transition and climate change response, wind turbine equipment is rapidly becoming larger, smarter, and greener. Onshore turbines are expanding from 2-4 MW to 5-6 MW, while offshore turbines are expanding from 6-8 MW to 10-14 MW. Consequently, the loads borne by towers are increasing. In particular, facing harsh, alternating loads such as waves, tides, and storms at sea places even higher demands on steel plates for their service safety, including low-temperature fracture toughness, fatigue resistance, and corrosion resistance. Currently, while 355 MPa-grade wind turbine steel plates with a thickness of less than 60 mm are widely used in wind turbine equipment, cracking in welds (especially girth welds) due to insufficient strength, leading to tower collapses, is a common occurrence. Therefore, to ensure the safe and reliable operation of wind turbine facilities, the trend toward higher strength grades for wind turbine steel plates is irreversible. The use of high-strength steel reduces energy consumption during the manufacturing process, while also enabling tower diameters to meet transportation restrictions, reducing transportation costs and fostering green development across the entire industry chain. Improving the toughness, fatigue resistance and weldability of steel plates, while also achieving excellent corrosion resistance and other comprehensive performance indicators, is an important way to meet the large-scale, green and economical development of wind power and the safety performance of tower services.

[0005] Currently, reports on steel plates for wind power generation are mostly focused on grades between 345 and 460 MPa. The production process involves TMCP (+tempering), normalizing, and normalizing 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 wind power steel, using a normalizing rolling process, with a strength of 420 MPa. Patent CN113969372B discloses a low-carbon, fatigue-resistant wind power steel plate and preparation method, with a yield strength of ≥465 MPa. Depending on the method, the maximum yield strength is 475 MPa. Clearly, none of the above technologies can meet the demand for 500 MPa-grade steel plates in the offshore wind power facility manufacturing industry.

[0006] Patent application CN116219320A discloses a lightweight pile steel for marine environment and its production method, the composition of which 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 the strength reaches 500MPa. Its composition system is relatively complex, and more precious metals such as Ni, Cr, and Mo are added. It is costly and will deteriorate welding performance, while failing to reflect the fatigue performance of the steel plate.

[0007] Patent application CN115896612A discloses a low-carbon equivalent weathering steel with a yield strength of 500 MPa and a production method thereof, comprising C: 0.09% to 0.12%, Si: 0.30% to 0.40%, Mn: 1.20% to 1.40%, P ≤ 0.015%, S ≤ 0.005%, Cr: 0.35% to 0.50%, Cu: 0.18% to 0.25%, Ni: 0.15% to 0.30%, and Ti: 0.055% to 0.075%. The steel has a high Ti content, which easily forms large TiN particles and fails to achieve the grain refinement effect. Furthermore, although the steel has certain atmospheric corrosion resistance, it cannot adapt to the service safety in the highly corrosive environment of seawater.

[0008] Summary of the Invention

[0009] To address the shortcomings and deficiencies of existing technologies, the present invention aims to provide a fatigue-resistant and corrosion-resistant 500MPa-grade steel plate for offshore wind power generation, and its preparation method. Produced using the TMCP+tempering process, the 500MPa-grade steel plate exhibits high strength, toughness, fatigue resistance, corrosion resistance, and ease of welding, meeting the requirements for safe service and high-strength, lightweight development of offshore wind power facilities.

[0010] To achieve the above objectives, the first aspect of the present invention provides a composition design of a fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate, which adopts the following technical solutions:

[0011] Disclosed is a fatigue-resistant and corrosion-resistant 500MPa-grade steel plate for marine wind power. The steel plate comprises the following chemical components by mass: 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%, Pcm is controlled to be ≤0.25%, and the remainder is Fe and unavoidable impurity elements.

[0012] Wherein: Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B, where the element symbols represent the mass percentage of the corresponding elements in the steel plate.

[0013] In the above-mentioned fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate, as a preferred embodiment, the unavoidable impurity elements, in terms of 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%.

[0014] In the above fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate, as a preferred embodiment, the thickness of the steel plate is 30mm to 60mm, the yield strength of the steel plate is ≥500MPa, the tensile strength is 600-760MPa, the elongation after fracture is ≥17%, the core impact energy at -30℃ is ≥120J, and the corrosion resistance is improved by more than 50% compared with the conventional 355MPa grade marine wind power steel plate. After the steel plate is welded by submerged arc welding with a heat input of ≥35kJ / cm, the tensile strength of the welded joint is ≥600MPa, and the impact energy at -30℃ is ≥80J; the welded joint is under the condition of a stress ratio (ratio of minimum loading stress to maximum loading stress) of 0.5, 10 7 The fatigue limit of cyclic axial loading is ≥450MPa.

[0015] The effects of the main alloys in the fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate are as follows:

[0016] Nickel is the element that most significantly improves the low-temperature toughness of steel sheets. Adding an appropriate amount of nickel can reduce the stacking fault energy of crystals, facilitate dislocation slip, and improve impact toughness. Furthermore, nickel promotes the formation of a dense, protective rust layer on the steel sheet's surface, enhancing its corrosion resistance. However, excessive nickel content is detrimental to weldability. Therefore, in the present invention, the nickel content is controlled to 0.28% to 0.38%.

[0017] Cu can improve the corrosion resistance and strength of steel, as well as improve weldability and machinability. However, excessive Cu content can increase the hot brittleness of the steel plate. Therefore, the Cu content of the present invention is controlled to be between 0.35% and 0.55%.

[0018] Nb can effectively refine grains and also contribute to precipitation strengthening; however, due to the limitation of C and the influence of heating temperature, too high a Nb content cannot fully dissolve. Therefore, in the present invention, the Nb content is controlled to 0.030% to 0.040%.

[0019] Ti also contributes to grain refinement and precipitation strengthening, 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 the niobium microalloying element. However, excessive titanium content can easily form large TiN particles, negating the grain refinement effect. Therefore, the present invention limits the Ti content to 0.018% to 0.030%.

[0020] Al can fix free nitrogen in steel, improving the low-temperature toughness of the steel plate and weld HAZ. 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 increases the number and size of inclusions in the steel, reduces the internal quality of the steel plate, and affects the hot working, welding, and machinability of the steel. Therefore, the present invention controls the Al content to 0.020% to 0.050%.

[0021] Pcm: Controlling the cold crack sensitivity coefficient is beneficial to ensuring the welding performance of the product. The present invention controls Pcm≤0.25%.

[0022] Impurity elements in steel, such as S and P, can increase the degree of segregation in continuous casting slabs, resulting in poor flaw detection after welding and deteriorating fatigue and corrosion resistance. Therefore, the S and P contents are controlled below 0.005% and 0.010%, respectively.

[0023] A second aspect of the present invention provides a method for preparing the above-mentioned fatigue-resistant and corrosion-resistant 500 MPa grade marine wind power steel plate, comprising the following steps:

[0024] 1) Hot metal pretreatment: The hot metal is desulfurized by the KR method to control the sulfur content of the hot metal to below 0.008%.

[0025] 2) Converter smelting: molten iron and scrap steel are subjected to primary smelting to obtain primary molten steel;

[0026] 3) Molten steel refining: LF refining + RH refining of the molten steel;

[0027] 4) Billet casting: Casting to obtain continuous casting billets;

[0028] 5) Billet heating: The billet is cold charged into the furnace; a multi-stage heating method is adopted;

[0029] 6) Controlled rolling: The ingot is subjected to two stages of rough rolling and finish rolling to obtain steel plates of specific dimensions;

[0030] 7) Controlled cooling: cooling the steel plate after rolling;

[0031] 8) Tempering: Tempering heat treatment is performed on the steel plate after water cooling.

[0032] In the above-mentioned method for preparing fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plates, as a preferred embodiment, in the molten iron pretreatment, the molten iron is desulfurized by the KR method to control the sulfur content of the molten iron to below 0.008%, and the desulfurization temperature is 1240℃~1300℃.

[0033] In the above-mentioned method for preparing fatigue-resistant and corrosion-resistant 500 MPa-grade steel plate for offshore wind power generation, as a preferred embodiment, during the converter smelting, molten iron after KR desulfurization treatment, scrap steel, nickel plate, and copper plate are added to the converter; preferably, the scrap steel / (molten iron + scrap steel) ratio is ≤10wt%. During smelting, the final slag basicity is controlled at R=3.0-4.0, and alloying is performed using metallic manganese, ferroniobium, and ferrosilicon. The alloy is added when 20%-30% of the steel is discharged and is completely added when the molten steel reaches 70%-80%. Preferably, ferroaluminum-manganese is added at 3-3.5 kg / t to deoxidize the molten steel.

[0034] This invention achieves low-C, low-Mn, and low-P composition control during converter smelting, aiming to improve C and Mn segregation in the steel plate and reduce inclusion levels, thereby enhancing the steel plate's impact toughness, fatigue strength, and corrosion resistance. Ni and Cu, which enhance strength and corrosion resistance, and Nb, which refines grains, must be properly incorporated during this stage to avoid adding them during subsequent refining, which could lead to incomplete dissolution and ineffective results.

[0035] In the above-mentioned method for preparing fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plates, as a preferred embodiment, in the molten steel refining step, the molten steel refining is divided into LF refining and RH refining. The LF refining is carried out with bottom blowing of argon and stirring throughout the process. Aluminum particles, calcium carbide, and silicon carbide are used to adjust the slag. The top slag before leaving the station is white slag, and the retention time is ≥10 minutes to reduce the S content; aluminum wire is used to adjust the Al composition to control the oxygen content of the molten steel; titanium element is extremely easy to combine with oxygen, and titanium wire is needed to adjust the titanium content to 0.018%-0.030% in this stage. Trace titanium can fix nitrogen in the steel and promote the solid solution of niobium element to refine the grains and improve the strength, toughness and fatigue resistance. RH refining ensures that the vacuum degree is ≤30Pa and the pure degassing time is ≥5min. After the RH refining is completed, 100-120m of nano high calcium wire is fed to each furnace. If the casting furnace is stopped, the length of the nano high calcium wire fed is ≥140m. The calcium treatment promotes the refinement and spheroidization of inclusions, which can effectively improve the impact toughness, corrosion resistance and fatigue performance of the steel; preferably, the LF refining time is ≥50min, of which the argon blowing time is ≥5min; preferably, the RH refining time is ≥50min, of which the soft argon blowing time is ≥14min.

[0036] In the above-mentioned preparation method of fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate, as a preferred embodiment, in the casting of the ingot, full-process protective casting is adopted, the crystallizer uses peritectic steel protective slag, and the tundish is covered with carbonized rice husks to prevent the oxidation of the molten steel and remove the inclusions floating on the molten steel; preferably, the superheat of the molten steel during casting is ≤15°C, and the ingot drawing speed is 0.80-0.10m / min; preferably, for steel plates with a specification of 30mm≤finished product thickness≤50mm, 250mm or 300mm thick ingots are selected; for steel plates with a specification of 50mm<finished product thickness≤60mm, 300mm thick ingots are selected; preferably, the ingots are put into the pit or stacked for slow cooling ≥48h.

[0037] In the above-mentioned method for preparing fatigue-resistant and corrosion-resistant 500MPa-grade steel plates for offshore wind power, as a preferred embodiment, during the heating of the slab, the slab is cold-charged into the furnace, and a multi-stage heating and heating method is adopted, with the first heating stage at 1050-1200°C, the second heating stage at 1180-1250°C, and the soaking stage at 1160-1220°C. The furnace pressure is well controlled, and a small amount of coal gas and air is used in the soaking stage to keep the temperature slightly lower than that of the second heating stage, 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°C. This ensures that the slab is evenly and thoroughly fired while preventing excessive growth of austenite grains. Preferably, after the slab is heated, a high-pressure water dephosphorization treatment is performed.

[0038] In the above-mentioned preparation method of fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate, as a preferred embodiment, in the controlled rolling step, the rolling process is a two-stage rolling of rough rolling and finish rolling; preferably, the rough rolling is recrystallization rolling, adopts a large reduction mode, the rough rolling passes ≤ 6, and ensures that the pass reduction rate of at least 2 passes (single pass thickness reduction / entry thickness) is ≥ 18%; preferably, in the rough rolling, for steel plates with a specification of 30mm≤finished product thickness<40mm, the intermediate billet thickness after rough rolling / finished product thickness = 3.5; for steel plates with a specification of 40mm≤finished product thickness<50mm, the intermediate billet thickness after rough rolling / finished product thickness = 3.0; for steel plates with a specification of 50mm≤finished product thickness≤60mm, the intermediate billet thickness after rough rolling / finished product thickness = 2.4. Preferably, the finishing rolling is non-recrystallization rolling, and the finishing rolling passes are ≤8, more preferably, the finishing rolling passes are 6; more preferably, in the finishing rolling, for steel plates with a specification of 30mm≤finished product thickness<45mm, the finishing rolling start temperature is 840-850℃; for steel plates with a specification of 45mm≤finished product thickness≤60mm, the finishing rolling start temperature is 830-840℃; more preferably, the last three deformation passes are carried out within the temperature range of 800-820℃.

[0039] The intermediate billet thickness in the present invention refers to the thickness of the cast billet after rough rolling. By limiting rough rolling to recrystallization rolling, with 6 or fewer rough rolling passes and ensuring a reduction ratio of 18% or greater in at least two passes, the effect of the Nb element is fully utilized. Austenite grains are fully refined through high reduction ratios, improving the strength, toughness, and fatigue resistance of the steel plate. By limiting finish rolling to non-recrystallization rolling, with 8 or fewer finishing passes, the cumulative deformation effect between passes and the forced phase transformation mechanism induced by defects within the deformed 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 strength, toughness, and fatigue resistance of the steel plate. The intermediate billet thickness obtained after rough rolling is controlled to rationally allocate the reductions for roughing and finishing rolling, thereby improving the overall performance of the steel plate.

[0040] In the above-mentioned method for preparing fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate, as a preferred embodiment, in the controlled cooling step, the start cooling temperature is controlled at 760-780°C, and the final cooling temperature is controlled at 500-530°C; preferably, the cooling rate is 10-12°C / s.

[0041] In order to ensure welding performance, the present invention adopts a low-carbon and low-manganese component design, and the amount of hardenability elements added is relatively low. Therefore, a strong water cooling process after rolling is selected to obtain a structure in which bainite accounts for more than 60%. While improving the strength of the steel plate, the micro-grain structure is refined to ensure the low-temperature impact performance and fatigue resistance of the steel plate.

[0042] In the above-mentioned method for preparing fatigue-resistant and corrosion-resistant 500MPa grade steel plates for marine wind power, as a preferred embodiment, in the tempering step, for steel plates with a specification of 30mm≤finished product thickness<50mm, the heating temperature is 420±5℃, and the holding time is 15±2min; for steel plates with a specification of 50mm≤finished product thickness≤60mm, the heating temperature is 440±5℃, and the holding time is 20±2min.

[0043] The present invention can effectively eliminate the internal stress of the steel plate after strong water cooling through tempering heat treatment, reduce the source of stress cracks, eliminate the corrosion point difference in different parts caused by uneven stress, and improve the fatigue resistance and corrosion resistance of the steel plate.

[0044] 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 structure of the steel plate obtained by the preparation method is about 60% bainite + about 30% acicular ferrite + retained austenite.

[0045] Compared with the prior art, the advantages of the present invention are:

[0046] (1) The present invention guarantees a long service life. The capacity of a single unit of an offshore wind turbine is generally more than twice that of an onshore wind turbine. The load borne by the tower is more complex, and seawater, as a highly corrosive medium, constantly threatens the service safety performance of the wind turbine. The present invention improves the C and Mn segregation of the steel plate, controls the P and S content, promotes grain refinement, improves the characteristics of the rust layer, and eliminates the internal stress of the steel plate through tempering treatment after controlled cooling, thereby inhibiting the generation and expansion of fatigue cracks. At the same time, it reduces the density of corrosion-active inclusions and the corrosion potential difference of the structure, making the fatigue strength of the steel plate after welding ≥450MPa, and the corrosion resistance is improved by more than 50% compared with conventional 355MPa-grade wind power steel plates. It can straighten the "back" of the offshore unit and provide a strong guarantee for achieving a safe service life of 20 years.

[0047] (2) The present invention achieves high welding efficiency. The present invention adopts a low-carbon, low-manganese, and low-carbon equivalent composition design, obtains high-quality ingots through high-clean molten steel smelting technology, and adopts the TMCP production process to achieve high-permeability rolling, giving full play to the effects of solid solution strengthening, precipitation strengthening, and fine grain strengthening. While obtaining good strength and toughness of the steel plate, it also ensures excellent welding performance. The welding heat input can reach more than 35kJ / cm, effectively realizing high efficiency of the processing process.

[0048] (3) The present invention achieves green and lightweight construction. Due to the increased strength, wind turbine towers manufactured using the product of the present invention can be reduced by more than 10% compared to conventional 355MPa offshore wind turbine panels. At the same time, the improved corrosion resistance of the steel plate can reduce the corrosion margin design of the steel plate when serving in a highly corrosive marine environment, effectively meeting the industry's demand for the strengthening of offshore wind power facilities. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG1 is a metallographic structure diagram of a steel plate according to Example 1 of the present invention. DETAILED DESCRIPTION

[0050] Unless otherwise specified, any feature disclosed in this specification may be replaced by an alternative feature having equivalent or similar purposes. Unless otherwise specified, each feature is merely an example of a series of equivalent or similar features. The description is intended solely to aid understanding of the present invention and should not be construed as a specific limitation of the present invention. The present invention will be further described in detail below using specific embodiments.

[0051] Example 1: The thickness of the steel plate is 30 mm.

[0052] The present invention provides a fatigue-resistant and corrosion-resistant 500MPa-grade steel plate for marine wind power. After KR treatment, the S content is 0.004%. During converter smelting, Ni, Cu elements, and Nb element for grain refinement must be properly matched to avoid supplementary addition during the subsequent refining process. The LF refining time is 50 minutes, during which titanium wire is fed to adjust the titanium content to 0.018%-0.030%. The RH refining time is 55 minutes, and 100m of nano high-calcium wire is fed after RH. The smelting chemical composition and weight percentage content 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%, and the rest are Fe and unavoidable impurity elements, Pcm=0.23%. After that, 250 mm thick continuous casting billets were cast at a billet drawing speed of 0.95 m / min. After the billets were cut, they were slowly cooled for 48 hours.

[0053] The ingot was heated after slow cooling, with a tapping temperature of 1190°C and a furnace time of 270 min. Rough rolling adopted a large reduction mode with a total of 6 passes. The reduction rates of the 5th and 6th passes were 18.3% and 20.7% respectively. The thickness of the intermediate billet after rough rolling was 105 mm. The finishing rolling stage began when the temperature reached 850°C, with 8 finishing passes. The rolling temperatures of the last 3 finishing passes were 815°C, 814°C and 810°C respectively. After rolling, controlled cooling was carried out at a rate of 12°C / s, with a starting cooling temperature of 778°C and a final cooling temperature of 503°C. The tempering heating temperature was 420°C and the holding time was 15 min.

[0054] Example 2: The thickness of the steel plate is 50 mm.

[0055] The present invention provides a fatigue-resistant and corrosion-resistant 500MPa-grade steel plate for marine wind power. After KR treatment, the S content is 0.006%. During converter smelting, Ni, Cu elements, and Nb element for grain refinement must be properly matched to avoid additional addition during the subsequent refining process. The LF refining time is 58 minutes, during which titanium wire is fed to adjust the titanium content to 0.018%-0.030%. The RH refining time is 55 minutes, and 110m of nano-high calcium wire is fed after RH. The smelting chemical composition and weight percentage content 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%, and the rest are Fe and unavoidable impurity elements, Pcm = 0.24%. After that, 250 mm thick continuous casting billets were cast at a billet drawing speed of 0.95 m / min. After the billets were cut, they were slowly cooled for 48 hours.

[0056] The ingot was heated after slow cooling, with a tapping temperature of 1200°C and a furnace time of 275 min. Rough rolling adopted a large reduction mode with a total of 5 passes. The reduction rates of the 4th and 5th passes were 19.1% and 21.2% respectively. The thickness of the intermediate billet after rough rolling was 120 mm. The finishing rolling stage began when the temperature reached 837°C. The finishing rolling was carried out for 8 passes. The rolling temperatures of the last 3 passes of the finishing rolling were 818°C, 815°C and 813°C respectively. After rolling, the billet was controlled to cool at a rate of 11°C / s, with a starting cooling temperature of 770°C and a final cooling temperature of 518°C. The tempering heating temperature was 440°C and the holding time was 20 min.

[0057] Example 3: The thickness of the steel plate is 60 mm.

[0058] The present invention provides a fatigue-resistant and corrosion-resistant 500MPa-grade steel plate for marine wind power. After KR treatment, the S content is 0.005%. During converter smelting, Ni, Cu elements, and Nb element for grain refinement must be properly matched to avoid additional addition during the subsequent refining process. The LF refining time is 55 minutes, during which titanium wire is fed to adjust the titanium content to 0.018%-0.030%. The RH refining time is 53 minutes, and 120m of nano-high calcium wire is fed after RH. The smelting chemical composition and weight percentage content 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%, and the rest are Fe and unavoidable impurity elements, Pcm = 0.24%. Then, a 300 mm thick continuous casting slab was cast, the slab was stretched at 0.80 m / min, and the slab was slowly cooled for 48 hours after cutting.

[0059] The ingot was heated after slow cooling, with a tapping temperature of 1182°C and a furnace time of 278 min. Rough rolling adopted a large reduction mode with a total of 5 passes. The reduction rates of 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 when the temperature reached 830°C, with 6 finishing passes. The rolling temperatures of the last 3 finishing passes were 820°C, 815°C and 812°C respectively. After rolling, the billet was controlled to cool at a rate of 10°C / s, with a starting cooling temperature of 769°C and a final cooling temperature of 521°C. The tempering heating temperature was 440°C and the holding time was 20 min.

[0060] Comparative Example 1: The steel plate thickness is 60 mm. The water cooling strength after rolling is relatively weak. The specific process is as follows.

[0061] The 300 mm thick continuous casting slab was obtained from the same furnace as in Example 3, with the same composition. The slab was heated to a tapping temperature of 1181°C, with a furnace time of 277 min. Rough rolling was performed in a high-reduction mode with five passes, with reductions of 19.0% and 21.1% for the fourth and fifth passes, respectively. The intermediate slab thickness after rough rolling was 144 mm. Finish rolling began after the temperature reached 830°C, with six passes, and the final three passes were at temperatures of 820°C, 816°C, and 812°C, respectively. After rolling, controlled cooling was performed at a rate of 9°C / s, with a starting cooling temperature of 770°C and a final cooling temperature of 548°C. Tempering was then performed at a temperature of 440°C with a holding time of 20 min.

[0062] Comparative Example 2: The steel plate has a thickness of 60 mm and is not subjected to tempering treatment after water cooling. The specific process is as follows.

[0063] Comparative Example 2 and Example 3 were produced from the same furnace, with the same composition and using 300mm thick continuous casting slabs. The slabs were heated to a tapping temperature of 1181°C, with a furnace time of 280 minutes. Rough rolling was performed using a high-reduction mode with five passes, with reductions of 19.1% and 20.8% in the fourth and fifth passes, respectively. The intermediate slab thickness after rough rolling was 144mm. Finish rolling commenced after the temperature reached 830°C, with six passes, the final three of which were at 820°C, 816°C, and 814°C, respectively. After rolling, controlled cooling was performed at a rate of 10°C / s, with the initial cooling temperature at 768°C and the final cooling temperature at 525°C. No subsequent tempering heat treatment was performed.

[0064] Table 1 shows the mechanical properties of the steel plate base material. In Examples 1-3, the yield strength is ≥500MPa, the tensile strength is 600-760MPa, the elongation after fracture is ≥17%, and the core impact energy at -30°C is ≥120J. The room temperature structure of the steel plate obtained by the preparation method is about 60% bainite + about 30% acicular ferrite + retained austenite. Figure 1 is a metallographic structure diagram of the obtained steel plate. In Comparative Example 1, due to the low water cooling intensity, the amount of ferrite and pearlite formed is relatively large, and the strength is significantly lower than that of the embodiment. The performance of the base material of Comparative Example 2 is similar to that of the embodiment.

[0065] The steel plates were welded by submerged arc welding with a heat input of 35.50 to 40.09 kJ / cm. The mechanical properties after welding are shown in Table 2. In Examples 1-3, the tensile strength of the welded joints was ≥600 MPa, the impact energy at -30°C was ≥80 J, the fatigue test stress ratio was 0.5, and the sine wave stress cycle was ≥10 7 The fatigue limit of the steel plate was ≥450 MPa. The post-weld strength of Comparative Example 1 decreased significantly, indicating that the ferrite + pearlite-based structure deteriorated significantly after welding at a heat input of 35 kJ / cm. Furthermore, due to its inherent low strength, the fatigue limit was also low. The post-weld strength of Comparative Example 2 was similar to that of the Example, but its fatigue limit was significantly lower, even lower than that of Comparative Example 1. This indicates that without tempering after water cooling, the internal stress of the steel plate easily triggered crack formation, reducing fatigue strength.

[0066] The steel plate was immersed in laboratory artificial seawater (composition as shown in Table 3) at a test temperature of 25±2°C for 30 days. The corrosion rate was calculated by the weight loss method. The results are shown in Table 4. The chemical composition of the TMCP state 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 more than 50% higher than that of the conventional 355MPa grade marine wind power steel plate.

[0067] Table 1 Mechanical properties of base material

[0068] Table 2 Mechanical properties after welding

[0069] Table 3 Composition of artificial seawater

[0070] Table 4 Corrosion rate in seawater

[0071] Finally, it should be noted that the above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art 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 are intended to be encompassed by 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 is as follows by mass percentage: 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 the rest are Fe and unavoidable impurity elements.

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

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

4. A method for preparing a fatigue-resistant and corrosion-resistant 500MPa grade steel plate for offshore wind power, comprising the following steps: 1) Hot metal pretreatment: The hot metal is desulfurized by the KR method to control the sulfur content of the hot metal to below 0.008%; 2) Converter smelting: molten iron and scrap steel are smelted to obtain primary molten steel; 3) Molten steel refining: LF refining + RH refining of molten steel; 4) Ingot casting: Casting to obtain continuous casting ingot; 5) Billet heating: cold charging of billets into the furnace; multi-stage heating method; 6) Controlled rolling: The ingot is subjected to two-stage rolling, namely rough rolling and finish rolling, to obtain a steel plate; 7) Controlled cooling: cooling the steel plate after rolling; 8) Tempering: Tempering heat treatment is performed on the steel plate after water cooling.

5. The method for preparing the fatigue-resistant and corrosion-resistant 500MPa grade steel plate for offshore wind power according to claim 4, characterized in that: In the step 1) of pre-treating the molten iron, the desulfurization temperature is 1240° C.-1300° C.; In the step 2) of converter smelting, the basicity of the final slag is controlled at R=3.0-4.0, and metal manganese, ferroniobium and ferrosilicon are used for alloying. The alloy is added when 20%-30% of the steel is discharged, and the addition is completed when the molten steel reaches 70%-80%. Aluminum-manganese-iron is added at 3-3.5 kg / t to deoxidize the molten steel.

6. The method for preparing the fatigue-resistant and corrosion-resistant 500MPa grade steel plate for offshore wind power according to claim 4, characterized in that: In the step 4), the superheat of the molten steel during casting is ≤15°C, and the casting speed is 0.80-0.10m / min.

7. The method for preparing the fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 4, characterized in that: In the step 5), the temperature of the first heating stage is 1050-1200°C, the temperature of the second heating stage is 1180-1250°C, the temperature of the soaking stage is 1160-1220°C, the soaking time is ≥50min, the time in the furnace is ≥10min / cm, and the tapping temperature is 1180-1200°C.

8. The method for preparing the fatigue-resistant and corrosion-resistant 500MPa grade marine wind power steel plate according to claim 4, characterized in that: In the step 6) of controlled rolling, the rough rolling is recrystallization rolling, a large reduction mode is adopted, the rough rolling passes are ≤6, and the reduction rate of at least 2 passes is ≥18%; the finishing rolling is non-recrystallization rolling, the finishing rolling passes are ≤8; for steel plates with a specification of 30mm≤finished product thickness<45mm, the finishing rolling start temperature is 840-850°C; for steel plates with a specification of 45mm≤finished product thickness≤60mm, the finishing rolling start temperature is 830-840°C; and the last 3 deformation passes are carried out in the temperature range of 800-820°C.

9. The method for preparing the fatigue-resistant and corrosion-resistant 500MPa grade steel plate for offshore wind power according to claim 4, characterized in that: In the step 7), the cooling is controlled such that the starting temperature is controlled at 760-780° C., the final temperature is controlled at 500-530° C., and the cooling rate is 10-12° C. / s.

10. The method for preparing the fatigue-resistant and corrosion-resistant 500MPa grade steel plate for offshore wind power according to claim 4, characterized in that: In the step 8) tempering, for steel plates with a specification of 30 mm ≤ finished product thickness < 50 mm, the heating temperature is 420 ± 5 ° C, and the holding time is 15 ± 2 min; for steel plates with a specification of 50 mm ≤ finished product thickness ≤ 60 mm, the heating temperature is 440 ± 5 ° C, and the holding time is 20 ± 2 min.

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