A thick-gauge fatigue-resistant and easy-to-weld high-strength steel plate for wind power and its preparation method

Abstract

The present invention provides a thick, fatigue-resistant, easily weldable, high-strength steel plate for wind power generation and a preparation method thereof. The steel plate comprises the following chemical composition by mass: C: 0.12% to 0.15%, Si: 0.15% to 0.35%, Mn: 1.30% to 1.50%, P ≤ 0.015%, S ≤ 0.005%, Nb: 0.015% to 0.030%, V: 0.015% to 0.030%, Ti: 0.005% to 0.020%, Al: 0.005% to 0.030%, with the remainder being Fe and unavoidable impurities. The preparation method comprises the steps of smelting, refining, continuous casting, heating, and rolling. The use of a low-C, high-Mn, Nb+V+Ti composite microalloying significantly inhibits austenite grain growth. The use of a normalizing rolling process results in a short process flow, low cost, and strong production line adaptability.

Description

Technical Field

[0001] The present invention relates to the field of iron and steel metallurgy, and in particular to a thick-gauge fatigue-resistant and easily welded high-strength steel plate for wind power and a preparation method thereof. Background Art

[0002] Wind energy is a clean, renewable energy source that is pollution-free, renewable, and has great potential. Wind energy offers numerous environmental benefits, including reducing local air pollution and water consumption. However, its most significant contribution is its reduction in carbon dioxide emissions. It is currently one of the most effective ways humanity can contribute to addressing global climate change.

[0003] The steel used in wind turbine towers is primarily low-alloy steel, requiring excellent strength, toughness, fatigue resistance, and weldability. Currently, wind power is expanding towards larger scale, with single offshore wind turbines exceeding 16MW and tower heights exceeding 100 meters. The tower base and foundation ring are critical stress-bearing areas, often welded from thick, high-strength wind turbine steel plates. This places extremely high demands on the steel's toughness and presents significant production challenges. The mechanical properties of high-strength wind turbine steel plates decrease significantly with increasing thickness, resulting in poor microstructure uniformity in the center and significant performance fluctuations.

[0004] Existing technologies mostly use the high-cost offline normalizing process for production, and lack welding and fatigue performance indicators, and have poor process adaptability.

[0005] Chinese patent application number CN202210210565.7 discloses a method for producing 80-100mm thick steel plates for wind power applications. Through a suitable smelting, continuous casting, heating, rolling, and controlled cooling process, the patent produces steel plates with excellent mechanical properties without requiring heat treatment, resulting in a low-cost process. However, the patent does not examine the weldability of the steel plates, lacking key data such as fatigue strength at the weld seam.

[0006] Chinese patent application number CN201710936745.2 discloses a particularly thick steel plate for wind power generation and its production method. Through rational composition design and electroslag remelting, this patent allows for the production of thick wind power steel plates ranging from 220 to 440 mm. This method offers advantages such as thicker products, but suffers from lower strength levels and a lack of research data on welding performance, particularly post-weld fatigue strength.

[0007] Chinese patent application number CN202111465116.9 discloses a thick, normalized, fatigue-resistant steel plate for wind power applications and its preparation method. The patent utilizes low-carbon micro-niobium technology and the addition of Cu, Ni, and Mo to improve the steel's strength and low-temperature impact resistance. However, the method described in the invention has a lengthy process flow, lacks data on impact toughness at extremely low temperatures (-60°C), and exhibits low post-weld fatigue strength.

[0008] In view of the above reasons, there is a need for a thick-gauge fatigue-resistant and easily welded high-strength steel plate for wind power and a preparation method thereof. Summary of the Invention

[0009] The purpose of the present invention is to provide a thick-gauge, fatigue-resistant, and easy-to-weld high-strength steel plate for wind power and a preparation method thereof. The preparation method has the characteristics of low cost and strong adaptability to production lines. The mechanical properties of the base material, post-weld mechanical properties and fatigue properties of the steel plate all meet application requirements.

[0010] In order to achieve the above object, the present invention provides the following technical solutions:

[0011] Disclosed is a thick, fatigue-resistant, and easily weldable high-strength steel plate for wind power generation. The steel plate comprises the following chemical components by mass percentage: C: 0.12% to 0.15%, Si: 0.15% to 0.35%, Mn: 1.30% to 1.50%, P≤0.015%, S≤0.005%, Nb: 0.015% to 0.030%, V: 0.015% to 0.030%, Ti: 0.005% to 0.020%, Al: 0.005% to 0.030%, and the remainder is Fe and unavoidable impurities.

[0012] Furthermore, in the above-mentioned thick-gauge fatigue-resistant and easily welded high-strength steel plate for wind power, the thickness of the steel plate is 70mm to 90mm, the yield strength of the parent material of the steel plate is ≥420MPa, the tensile strength is ≥540MPa, A ≥40%, the core impact energy at -60℃ is ≥200J, and CEV ≤0.41; the steel plate can be welded under the condition of a large wire input of ≥30kJ / cm. After the steel plate is welded, the tensile strength at the joint is ≥540MPa, the core impact energy at -60℃ in the heat-affected zone is ≥120J, the stress ratio at the welded joint is 0.5, and the cycle is 5×10 7 The ultimate fatigue stress under these conditions is ≥400MPa.

[0013] On the other hand, a method for preparing the above-mentioned thick-gauge fatigue-resistant and easily welded high-strength steel plate for wind power is provided, comprising the following steps:

[0014] Smelting: using converter for smelting;

[0015] Refining, refining includes LF refining and RH refining,

[0016] Continuous casting: using a continuous casting mold to produce slabs with a thickness of ≥300mm, and then subjecting the slabs to a slow cooling treatment after continuous casting;

[0017] Heating: the slab is heated by cold charging;

[0018] Rolling includes rough rolling and finishing rolling. Rough rolling is carried out first. The intermediate billet after rough rolling is cooled by a MULPIC water cooler and then transported to the finishing mill for finishing rolling. It then enters the straightening mill for straightening to obtain the finished steel plate.

[0019] Furthermore, in the above-mentioned preparation method, in the smelting step, the converter adopts a one-time carbon pulling and single slag process for smelting, the final slag basicity R is controlled between 3.2 and 4.2, a red clean ladle is used, the ladle temperature is ≥800°C, the steel release time is ≥3 minutes, high-sensitivity slag blocking is used for steel tapping, 1.7kg / t~3.2kg / t of aluminum manganese iron is used for deoxidation, low-carbon and low-phosphorus silicon manganese, ferrosilicon, ferroniobium, and ferrovanadium are added in batches, and synthetic slag and pre-melted slag are added along the steel flow when tapping, and the ratio of synthetic slag to pre-melted slag is 2:1.

[0020] Furthermore, in the above-mentioned preparation method, argon is blown bottom and stirred throughout the LF refining process, aluminum particles and aluminum slag are deoxidized, lime is added for slag formation, the final slag basicity is ≥2.7, silicon manganese, ferrosilicon, ferroniobium, and ferrovanadium alloys are used for composition fine-tuning, titanium wire and ferrotitanium are used to adjust the Ti composition; aluminum wire is used to adjust the Al composition, and the LF refining treatment time is controlled to be ≥40 minutes; after the LF refining is completed, RH refining is carried out, and the RH refining time is ≥45 minutes. After the RH refining treatment is completed, 85m to 160m of calcium aluminum wire is fed, and then soft blowing is carried out for more than 9 minutes.

[0021] Furthermore, in the above-mentioned preparation method, in the continuous casting step, the continuous casting machine pulling speed is 0.8m / min~1.1m / min, peritectic steel protective slag is used, the crystallizer vibration mode is a non-sinusoidal vibration mode, the secondary cooling water and dynamic light pressure are controlled according to the peritectic steel, and the slab enters the third cooling machine for water cooling after fire cutting to achieve shallow surface quenching of the slab surface. During the quenching process, the upper and lower water volume difference is controlled to be 15%~30%, and the surface temperature of the slab is controlled to be ≤600℃. After the surface completes the γ-α transformation, the surface cooling water is blown away; the slab uses waste heat for self-tempering, the self-tempering time is ≥120min, and after the self-tempering is completed, it enters the pit for slow cooling, and the slow cooling time is not less than 56 hours.

[0022] Furthermore, in the above-mentioned preparation method, in the heating step, the slab charging temperature is ≤350°C, the temperature of the soaking section of the heating furnace is controlled to be 1180°C~1260°C, the steel tapping temperature is 1170°C~1250°C, and the heating rate is 9min / cm~11.5min / cm; preferably, the temperature of the soaking section of the heating furnace is 1250°C, the steel tapping temperature is 1200°C, and the heating rate is 10min / cm.

[0023] Furthermore, in the above-mentioned preparation method, in the rolling step, the rough rolling adopts a large reduction mode, and the rough rolling stage is completed by 4+1 passes or 6+1 passes according to the target steel plate thickness, wherein the +1 pass is an empty pass; the reduction rate of the rolling passes is controlled to increase gradually, the reduction rate of the first pass is ≥6%, and the reduction rate of the last pass in the 4+1 pass and 6+1 pass rolling modes should be ≥20%, and the pass reduction rate gradient is increased by 3% to 8%.

[0024] Furthermore, in the above preparation method, when a 300mm slab is used to produce a 90mm steel plate, the rough rolling passes are 5 passes, the first pass reduction rate is 7%, the last pass reduction rate is 27.5%, and the gradient increments are 5%, 7%, and 8.5%, respectively.

[0025] Furthermore, in the above-mentioned preparation method, in the rolling step, the finishing rolling adopts the normalizing rolling mode, and the control pass is 6+1 mode, wherein the +1 pass is an empty pass, and the reduction rate is gradually reduced. When the thickness of the finished product is 60mm~80mm, the first pass reduction rate is ≥12%, and when the thickness of the finished product is 80mm~90mm, the first pass reduction rate is ≥14%, and the reduction rate decreases by 0.5%~4% from pass to pass; preferably, when using 300mm slab to produce 90mm steel plate, 6+1 finishing rolling passes are adopted, the first pass reduction rate is 16.5%, and the reduction rate decreases by 1.0%~3.5% from pass to pass, and the reduction rate at the 6th pass is 5.8%.

[0026] Analysis shows that the present invention discloses a thick-gauge, fatigue-resistant, and easily weldable high-strength steel plate for wind power and its preparation method. The preparation method adopts low-C, high-Mn, Nb+V+Ti composite microalloying to form submicron-level Ti(C, N), V(C, N) and other precipitates, which significantly inhibit the growth of austenite grains and provide a structural basis for the subsequent rolling process. The Cottrell and Snoek gas masses formed by the Nb\Ti(C, N) composite nanocarbides formed in the finishing rolling stage can significantly hinder the movement of dislocations and pin the grain boundaries, inhibiting the growth of austenite grains. At the same time, due to the addition of Nb, V, and Ti microalloying elements, the upper limit of the non-recrystallization temperature is increased, the rolling process range is expanded, and the probability of mixed crystals in the two-phase region is reduced, which is conducive to improving strength and toughness. The slab triple cooling equipment quenching process is adopted to achieve shallow surface quenching of the slab, forming a slab shell with uniform thickness on the surface, achieving rapid cooling of the slab and defect-free preparation of the slab surface, and then using the slab residual heat self-tempering effect to refine the surface grains of the cast slab. Rough rolling utilizes a gradient-increasing reduction rate mode, while finishing rolling utilizes a gradient-decreasing reduction rate mode to fully refine grains and ensure effective rolling penetration. MULPIC cooling of the intermediate bar before finishing rolling reduces the intermediate bar's holding time, while the normalizing effect maximizes the dynamic recrystallization and grain refinement achieved during the roughing stage. The normalizing rolling process, used to produce thick wind power steel plates, achieves normalizing during the rolling process, eliminating the need for offline normalizing. This results in a short process flow, low costs, and strong production line adaptability, promising broad prospects for expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings and the accompanying drawings, which constitute part of this application, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. Among them:

[0028] Figure 1 Steel plate welding pass diagram of Example 1 of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments. Each example is provided by way of explanation of the present invention and is not intended to limit the present invention. Indeed, it will be apparent to those skilled in the art that modifications and variations may be made in the present invention without departing from the scope or spirit of the present invention. For example, a feature shown or described as part of one embodiment may be used in another embodiment to produce yet another embodiment. Therefore, it is intended that the present invention encompasses such modifications and variations as come within the scope of the appended claims and their equivalents.

[0030] According to an embodiment of the present invention, a thick-gauge fatigue-resistant and easy-to-weld high-strength steel plate for wind power is provided, which consists of the following chemical components by mass percentage: C: 0.12% to 0.15%, Si: 0.15% to 0.35%, Mn: 1.30% to 1.50%, P≤0.015%, S≤0.005%, Nb: 0.015% to 0.030%, V: 0.015% to 0.030%, Ti: 0.005% to 0.020%, Al: 0.005% to 0.030%, and the rest is Fe and unavoidable impurities.

[0031] Carbon: Carbon atoms in steel produce significant interstitial solid solution strengthening, significantly increasing the steel's strength and hardness. This makes it the most cost-effective strengthening method for steel materials. However, a carbon content exceeding 0.15% increases carbides in the steel, significantly reducing properties such as toughness and elongation, and also deteriorating weldability. Therefore, the carbon content in this invention is controlled between 0.12% and 0.15%.

[0032] Silicon: Si can be used as a deoxidizing element during the smelting process to increase the steel's antioxidant properties. At the same time, the incorporation of Si into the steel can cause lattice distortion and improve the strength of the ferrite structure in the steel. However, a silicon content exceeding 0.35% may cause the ferrite structure in the steel to become brittle, reducing the plasticity and toughness of the steel. Therefore, the silicon content of the present invention is controlled between 0.15% and 0.35%.

[0033] Manganese: Mn can lower the γ-α phase transformation temperature of steel, refine the pearlite lamellae in the steel, and increase the pearlite strength in low-carbon steel without significantly reducing ductility. Mn can control the content of oxides and sulfides during the smelting process, making the steel structure uniform and refined, and improving the strength and toughness of the steel. However, a Mn content exceeding 1.50% is prone to rolling cracking and deteriorates the weldability of the steel. Therefore, the manganese content of the present invention is controlled between 1.30% and 1.50%.

[0034] Phosphorus: Phosphorus is a readily segregating element in steel, making it susceptible to cold brittleness. Furthermore, phosphorus can easily cause weld cracks in welds. To ensure sufficient toughness in the weld metal, the phosphorus content in this invention is controlled below 0.015%.

[0035] Sulfur: Sulfur is a harmful element. It easily combines with manganese to form MnS inclusions, which deform during rolling and reduce the impact toughness of the steel. Sulfur also increases the hot brittleness of the weld metal, easily causing hot cracks and pores in the weld. Therefore, the sulfur content in this invention is controlled to below 0.005%.

[0036] Niobium: Nb promotes grain refinement and improves the toughness of steel. Furthermore, Nb combines with carbon and nitrogen in the steel to form nano-sized Nb(C,N), which significantly strengthens the steel and improves the mechanical properties of the heat-affected zone (HAZ). However, a niobium content exceeding 0.03% degrades weldability and cold deformation. Therefore, the niobium content in this invention is controlled between 0.015% and 0.030%.

[0037] Vanadium (V) forms a continuous solid solution with Fe, significantly reducing the austenite phase while simultaneously refining the ferrite grains, resulting in a significant grain refinement strengthening effect. V has a strong affinity with carbon, nitrogen, and other metals, forming VC and VN precipitates in the steel, which can produce a significant precipitation strengthening effect. V is a scarce resource, and its usage should be minimized. Therefore, the V content in this invention is controlled between 0.015% and 0.030%.

[0038] Titanium: As a good deoxidizing element in steel, Ti combines with carbon in steel to form stable TiC, which prevents the growth and coarsening of steel grains and improves the low-temperature impact toughness of steel. However, if the Ti content exceeds 0.02%, the content and size of titanium carbide particles increase, and the strength and toughness of the steel decrease. Therefore, the Ti content of the present invention is controlled between 0.005% and 0.020%.

[0039] Aluminum: Al is a strong deoxidizing element that can also refine grains and improve the toughness of steel. Controlling the Al content can improve the purity and fatigue strength of the steel. However, if the Al content exceeds 0.030%, it will increase the amount of alumina inclusions in the steel and deteriorate the mechanical properties of the steel. Therefore, the Al content of the present invention is controlled between 0.005% and 0.030%.

[0040] The design ideas of the present invention are as follows:

[0041] By adopting low C and high Mn, Nb+V+Ti composite microalloying, submicron-sized Ti(C, N), V(C, N) and other precipitates are formed, which significantly inhibit the growth of austenite grains and provide a structural basis for the subsequent rolling process; the Cottrell and Snoek gas clouds formed by the Nb\Ti(C, N) composite nanocarbides formed in the finishing rolling stage can significantly hinder the movement of dislocations and pin the grain boundaries, inhibiting the growth of austenite grains. At the same time, due to the addition of Nb, V, and Ti microalloying elements, the upper limit of the non-recrystallization temperature is increased, the rolling process range is expanded, and the probability of mixed crystals in the two-phase region is reduced, which is beneficial to improving strength and toughness.

[0042] The unrecrystallized zone refers to the high-strength low-alloy steel (HSLA) containing alloying elements such as Nb. The recrystallization temperature rises, generally reaching around 950°C. Therefore, the region where recrystallization does not occur between 700°C and 950°C, the temperature at which ordinary carbon steel recrystallizes, is called the unrecrystallized zone. Recrystallization of deformed austenite during hot rolling, particularly dynamic recrystallization, refines the austenite grains (recrystallization-controlled rolling), forming equiaxed austenite grains. Microalloying particles such as V (C, N) can form in large quantities within the austenite, becoming polygonal ferrite nucleation sites. During the rolling phase of the unrecrystallized zone, the austenite grains are significantly elongated. During this stage, the Cottrell and Snoek gas masses formed by Nb\Ti (C, N) composite nanocarbides significantly hinder dislocation movement and pin grain boundaries, inhibiting austenite grain growth. This increases the location and rate of ferrite nucleation during the phase transformation, refines the grains, and enhances the fine-grain strengthening effect. During the subsequent cooling process of the steel plate, Nb\Ti(C, N) composite nano-carbides will continue to disperse and precipitate or interphase, hindering dislocation movement and producing a precipitation strengthening effect.

[0043] The present invention also discloses a method for preparing the above-mentioned thick-gauge fatigue-resistant and easily welded high-strength steel plate for wind power, comprising the following steps:

[0044] S1 smelting

[0045] The converter is used for smelting. High-quality lime and dolomite are used in the converter. The high gun position and charging timing are controlled during the smelting process. The slag is added 3 minutes before the end point. The converter adopts a one-time carbon pulling and single slag process for smelting. The final slag basicity R is controlled between 3.2 and 4.2 (for example: 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2) to achieve early slag formation in the early stage and good slag formation during the process. A red clean ladle is used, the ladle temperature is ≥800℃, and the steel discharge time is ≥3 minutes. A high-sensitivity slag blocking system is used for steel tapping to prevent large amounts of slag. Deoxidation and alloying: Use 1.7kg / t to 3.2kg / t (e.g. 1.7kg / t, 1.9kg / t, 2.0kg / t, 2.2kg / t, 2.4kg / t, 2.6kg / t, 2.8kg / t, 3.0kg / t, 3.2kg / t) of ferroaluminum manganese steel for deoxidation. When the molten steel is one-quarter full, add low-carbon and low-phosphorus silicon manganese, ferrosilicon, ferroniobium and ferrovanadium in batches. Add all the molten steel when the molten steel is three-quarters full. When tapping, add synthetic slag and pre-melted slag along the steel flow. The ratio (by mass) of synthetic slag to pre-melted slag is 2:1 (e.g. 300kg synthetic slag + 150kg pre-melted slag).

[0046] S2: Refining

[0047] Refining involves both LF refining and RH refining. During LF refining, argon is blown throughout the entire process for bottom stirring. Molten steel must not be exposed to the elements to prevent secondary oxidation. Aluminum pellets and aluminum slag are used for deoxidation, and weak argon stirring is employed during the deoxidation process. Lime is added to form slag, and the top slag must be yellow-white or white before leaving the station. This slag must be held for ≥13 minutes, and the final slag basicity must be ≥2.7. Fine-tuning of the composition is performed using alloys such as silicon-manganese, ferrosilicon, ferroniobium, and ferrovanadium. Titanium wire and ferrotitanium are used to adjust the Ti content, while aluminum wire is used to adjust the Al content. The LF refining process time is controlled to be ≥40 minutes.

[0048] After LF refining is completed, RH refining is carried out. During RH refining, avoid chemical heating and ensure that the pure degassing time is ≥5 minutes. Control the RH refining time to ≥45 minutes. After the RH refining is completed, feed 85 meters to 160 meters of calcium aluminum wire (for example: 85 meters, 90 meters, 95 meters, 100 meters, 105 meters, 110 meters, 115 meters, 120 meters, 125 meters, 130 meters, 135 meters, 140 meters, 145 meters, 150 meters, 155 meters, 160 meters) to treat the inclusions with calcium, and then soft blow for more than 9 minutes.

[0049] S3: Continuous Casting

[0050] Slabs with a thickness of 300mm or greater are produced using a continuous casting mold. Casting speeds range from 0.8m / min to 1.1m / min, using peritectic steel protective slag. The mold vibration mode is non-sinusoidal. Secondary cooling water and dynamic soft reduction are controlled according to peritectic steel. After fire cutting, the slabs enter the tertiary cooling machine for water cooling. During this stage, the equipment operates at maximum water flow and valve opening to achieve a shallow surface quenching of the slab. Because water from the upper nozzles retains the steel plate, while water from the lower nozzles falls into the water collection pipe, the lower water flow is controlled to exceed the upper flow during quenching, with a difference of 15% to 30%. The slab surface temperature is maintained at ≤600°C, and the surface cooling water is blown off after the surface completes the γ-α transformation. The slabs undergo auto-tempering using residual heat for ≥120 minutes. After auto-tempering, they are placed in a pit for slow cooling, which lasts for at least 56 hours. The slab triple cooling equipment quenching process is adopted to achieve shallow surface quenching of the slab, forming a slab shell with uniform thickness on the surface, realizing rapid cooling of the slab and defect-free preparation of the slab surface, and then utilizing the self-tempering effect of the slab residual heat to refine the surface grains of the casting.

[0051] S4: Heating

[0052] The slab is heated by cold charging, and the charging temperature of the slab is ≤350℃. Since this steel grade adopts low alloy and low CEV design (low CEV is a relative concept, usually less than 0.45 is considered as low CEV), in order to prevent coarse grains, low temperature steel burning is required. The temperature of the soaking section of the heating furnace is controlled to be 1180℃~1260℃ (for example: 1180℃, 1190℃, 1200℃, 1210℃, 1220℃, 1230℃, 1240℃, 1250℃, 1260℃), and the tapping temperature is 1170℃~1250℃ (for example: 1170℃, 11 The above heating rates and temperatures can homogenize the original austenite structure in the slab, fully dissolve the alloying elements such as Nb, V, and Ti in the steel, and provide a basis for the subsequent precipitation of microalloy carbides in the steel. Preferably, the soaking zone temperature of the heating furnace is 1250°C, the tapping temperature is 1200°C, and the heating rate is 10 min / cm.

[0053] S5: Rolling

[0054] Rolling includes rough rolling and finishing rolling. Rough rolling is carried out first. The intermediate billet after rough rolling is cooled by a MULPIC water cooler, then transported to the finishing mill for finishing rolling, and then enters the straightening machine for straightening to obtain the finished steel plate.

[0055] Among them, the steel plate is rough rolled in a large reduction mode, and the intermediate billet after rough rolling is sent to the MULPIC water cooler for cooling. The intermediate billet is controlled to be reciprocatingly water-cooled in the water cooler through the non-rolling process. When the surface temperature of the intermediate billet is ≤830℃, a surface quenching zone of sufficient thickness is generated.

[0056] The rough rolling stage is completed in 4+1 or 6+1 passes based on the target steel plate thickness, with the +1 pass being an idle pass and no rolling force applied. In terms of deformation, the reduction rate of each rolling pass is controlled to increase gradually, with the first pass reduction rate ≥6%. The final pass reduction rate of both the 4+1 and 6+1 rolling modes should be ≥20%, with the reduction rate increasing gradually by 3% to 8% (for example: 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%). The temperature in the rough rolling stage is in the recrystallization zone, and the gradually increasing reduction rate can fully break up the austenite grains, promote complete recrystallization of the core structure, and greatly refine the austenite grains. Preferably, when a 90mm steel plate is produced using a 300mm slab, the rough rolling passes are 5, the first pass reduction is 7%, the last pass reduction is 27.5%, and the gradient increments are 5%, 7%, and 8.5%, respectively.

[0057] The specific finishing rolling process is shown in Table 1. During the finishing rolling stage, the core of the intermediate slab remains in a low-temperature, unrecrystallized zone. The residual heat from the core of the slab causes the surface to redden, achieving a normalizing effect. A gradient reduction reduction pattern is employed during the finishing rolling stage. The number of passes is controlled based on the finished product thickness, using a 6+1 pattern, with the +1 pass being an empty pass. For finished product thicknesses of 60mm-80mm, the first-pass reduction is ≥12%. For finished product thicknesses of 80mm-90mm, the first-pass reduction is ≥14%. The reduction decreases by 0.5%-4% with each subsequent pass. This gradient reduction in deformation during the finishing rolling stage ensures effective penetration during the high-temperature core rolling stage, improving microstructure uniformity in the core. Furthermore, the nanoparticles of NbC, TiC, and V (C, N) precipitated during the finishing rolling process pin the austenite grain boundaries, inhibiting abnormal austenite grain growth. Preferably, when a 90mm steel plate is produced using a 300mm slab, 6+1 passes of finishing rolling are used, with a first pass reduction rate of 16.5%, and a reduction rate that decreases from 1.0% to 3.5% pass by pass. The deformation (reduction rate) at the 6th pass can still reach 5.8%. After rolling, the steel plate enters the straightening machine for 1 or 3 passes to straighten the head and tail shapes of the steel plate and improve the yield rate. Finishing rolling adopts a normalizing rolling process to achieve normalizing treatment of the steel plate during the rolling process, eliminating the need for an offline normalizing process, resulting in a short process flow, low cost, and strong adaptability of the production line. Rough rolling adopts a gradient increase mode of reduction rate, and finishing rolling adopts a gradient decrease mode of reduction rate to fully refine the grains and ensure the rolling penetration effect. The MULPIC cooling of the intermediate billet before finishing rolling can reduce the waiting time of the intermediate billet. At the same time, the normalizing effect can retain the dynamic recrystallization and grain refinement effect of the rough rolling stage to the greatest extent.

[0058] Table 1: Variation of finishing rolling temperature and pass reduction rate

[0059]

[0060]

[0061] The thick wind power steel plates with a thickness of 70mm to 90mm rolled by the above method have the characteristics of high strength, toughness and low temperature resistance. The base material yield strength of the steel plates is ≥420MPa, tensile strength is ≥540MPa, A is ≥40%, core impact energy at -60℃ is ≥200J, and CEV is ≤0.41.

[0062] Steel plates can be welded under conditions of a maximum input of ≥30kJ / cm. Preheating of the steel plates before welding is not required. Gas shielded welding or submerged arc welding is used, with a single-sided V-groove. The interlayer temperature is controlled at ≤180°C, and the number of passes is determined based on the thickness of the steel plate. After welding, the tensile strength of the joint is ≥540MPa, the impact energy of the heat-affected zone at -60°C is ≥120J, the stress ratio of the weld joint is 0.5, and the cycle is 5×10 7 The ultimate fatigue stress under certain conditions is ≥400MPa, which fully meets the needs of high-energy-density welding.

[0063] Example 1:

[0064] A thick, fatigue-resistant, easily weldable high-strength steel plate for wind power generation, comprising the following chemical composition: C: 0.13%, Si: 0.30%, Mn: 1.41%, P: 0.010%, S: 0.003%, Nb: 0.025%, V: 0.027%, Ti: 0.017%, Al: 0.023%, with the remainder being Fe and unavoidable impurities. The steel plate is 80 mm thick, has a base metal yield strength of 457 MPa, a tensile strength of 562 MPa, an A content of 43%, an average core impact energy at -60°C of 217 J, and a CEV of 0.37. After automatic welding using a 30 kJ / cm2 linear input device, the joint has a tensile strength of 552 MPa, a core impact energy of ≥120 J at -60°C in the heat-affected zone, and a welded joint with a stress ratio of 0.5 and a cycle strength of 5×10 7 The ultimate fatigue stress under these conditions is 414 MPa, which fully meets the needs of high-energy-density welding.

[0065] The method for preparing the thick, fatigue-resistant, and easily welded high-strength steel plate for wind power generation includes the following steps: molten iron is subjected to converter smelting, LF refining, and RH degassing refining before continuous casting. A 300mm thick continuous casting billet is selected, and the slab is subjected to slow cooling after continuous casting. During the converter smelting process, the converter stage adopts a single carbon pulling and single slag process, and the final slag basicity is controlled at R=3.5; the final slag basicity of the LF refining is 2.7, and the holding time is 15 minutes. During the RH refining treatment, 115m of calcium aluminum wire is fed, followed by soft blowing for 10 minutes, and the RH refining time is 48 minutes. The continuous casting process implements a 300mm billet shape, with a drawing speed of 0.9mm / min. During the three cooling stages, the slab surface is shallowly quenched, and the upper and lower water volume difference during quenching is controlled at 22.37%, and the slab surface temperature is 602°C. The slab is then self-tempered using residual heat for 132 minutes. After the self-tempering is completed, the slab is slowly cooled in a pit for 60 hours.

[0066] After heating and rolling, the slabs were converted into 80mm thick high-strength steel plates for wind power generation. During the heating process, the slab soaking zone temperature was 1195°C, the tapping temperature was 1189°C, and the heating rate was 10 min / cm. The rough rolling phase involved a 4+1 pass process, with the reduction ratio gradually increasing from 6.7% in the first pass to 23.90% in the final pass. The rolling deformation increased in steps, ranging from 4.8% to 6.6%. The finishing rolling phase involved a 6+1 pass process, with the reduction ratio decreasing with each pass. The first pass had a reduction of 16.5%, while the sixth pass had a reduction of 10.8%. The reduction ratio then decreased with each pass, ranging from 0.5% to 1.8%. The deformation ratios for each rolling pass are shown in Table 2.

[0067] Table 2: Deformation rate of rolling pass in Example 1 (%)

[0068]

[0069] After the steel plate prepared by the above preparation method is welded by submerged arc welding (SAW), the tensile strength of the weld joint is 552MPa, and the average impact energy of the core of the weld fusion metal area at -60℃ is 176J, indicating excellent comprehensive mechanical properties of the weld. 7 The fatigue limit strength of cyclic axial loading is 414MPa. The welding process is as follows Figure 1 , as shown in Table 3, and the mechanical properties are shown in Table 4.

[0070] Table 3: Steel plate submerged arc welding parameters of Example 1

[0071] Welding methods Submerged Arc Welding / SAW Welding equipment and model DC-100 automatic welding system Welding heat input 30kJ / cm Current / A 550±20 Voltage / V 34±2 Welding speed cm / min 38±2 Preheating temperature / ℃ 100-200 To this temperature / ℃ 100-200

[0072] Table 4: Mechanical properties and fatigue properties of the steel plate after welding in Example 1

[0073]

[0074] Example 2:

[0075] A thick, fatigue-resistant, easily weldable high-strength steel plate for wind power generation, comprising the following chemical composition: C: 0.14%, Si: 0.32%, Mn: 1.45%, P: 0.007%, S: 0.001%, Nb: 0.029%, V: 0.030%, Ti: 0.015%, Al: 0.025%, with the remainder being Fe and unavoidable impurities. The steel plate is 90 mm thick, has a base metal yield strength of 443 MPa, a tensile strength of 562 MPa, an A content of 40.2%, an average core impact energy of 183 J at -60°C, and a CEV of 0.39. After automatic welding using a 30 kJ / cm2 linear input device, the joint has a tensile strength of 552 MPa, a core impact energy of ≥120 J at -60°C in the heat-affected zone, and a stress ratio of 0.5 and a cycle of 5×10 7 The ultimate fatigue stress under certain conditions is ≥400MPa, which fully meets the needs of high input energy welding.

[0076] The preparation method of the above-mentioned thick-gauge fatigue-resistant and easy-to-weld high-strength steel plate for wind power includes the following steps: molten iron is smelted in a converter, refined by LF, and refined by RH degassing, and then continuously cast; a 300mm thick continuous casting billet is selected; and the slab needs to be slowly cooled after continuous casting. During the converter smelting process, the converter stage adopts a one-time carbon pulling and single slag process, and the final slag basicity is controlled at R=3.5; the LF refining final slag basicity is 2.7, the holding time is 17 minutes, and during the RH refining treatment, 132m of calcium aluminum wire is fed to treat the inclusions with calcium, followed by soft blowing for 9 minutes, and the RH refining time is 50min; the continuous casting process executes a 300mm billet shape, the pulling speed is 0.9mm / min, and the surface of the slab is shallowly quenched in the three cooling stages. During the quenching process, the difference between the upper and lower water volumes is controlled to be 20.17%, and the surface temperature of the slab is 587°C. Subsequently, the slab is self-tempered using waste heat, and the self-tempering time is 150min. After the self-tempering is completed, the slab is put into the pit for slow cooling, and the slow cooling time is 62 hours.

[0077] After heating and rolling, the slabs were converted into 90mm thick high-strength steel plates for wind power generation. During the heating process, the slab soaking zone temperature was 1200°C, the tapping temperature was 1187°C, and the heating rate was 10 min / cm. The rough rolling phase of the steel plate was completed using a 4+1 pass process, with the reduction ratio gradually increasing from 7.0% in the first pass to 27.5% in the fourth pass. The rolling deformation was increased in a stepwise manner, ranging from 5.0% to 8.5%. The finishing rolling phase required 6+1 passes, again using a pass-by-pass reduction method. The first pass had a reduction of 16.5% and the sixth pass had a reduction of 5.8%. The reduction ratio decreased with each pass, ranging from 1.0% to 3.5%. The deformation ratios for each rolling pass are shown in Table 5.

[0078] Table 5: Deformation rate of rolling pass in Example 2 (%)

[0079]

[0080] The welding equipment and parameters were the same as those in Example 1. After the steel plate prepared by the above preparation method was welded by submerged arc welding (SAW), the tensile strength of the weld joint was 546 MPa, and the average impact energy at -60°C in the center of the weld fusion metal area was 157 J, indicating excellent comprehensive mechanical properties of the weld. 7 The fatigue limit strength after cyclic axial loading is 420 MPa. The mechanical properties are shown in Table 6.

[0081] Table 6: Mechanical properties and fatigue properties of the steel plate after welding in Example 2

[0082]

[0083] Comparative Example 1

[0084] A thick, fatigue-resistant, easily weldable, high-strength steel plate for wind power generation, comprising the following chemical composition: C: 0.14%, Si: 0.30%, Mn: 1.40%, P: 0.008%, S: 0.003%, Nb: 0.027%, V: 0.026%, Ti: 0.020%, Al: 0.018%, with the remainder being Fe and unavoidable impurities. The steel plate has a thickness of 80 mm.

[0085] The method for preparing the steel plate comprises the following steps:

[0086] The implementation process of smelting, refining, continuous casting and heating is consistent with that of Example 1.

[0087] During the rolling process, the rough rolling stage of the steel plate is completed in 4+1 passes. The reduction ratio of each pass is controlled according to the rolling mill conditions, with the first pass reduction ratio being approximately 8% and the final pass reduction ratio being approximately 20%. The rolling deformation is dynamically adjusted according to the slab widening conditions and the rolling mill status. The finishing rolling stage requires 6+1 rolling passes, with the first pass reduction ratio being approximately 10% and the sixth pass reduction ratio being 8%. The reduction ratio is similar to that of rough rolling and is dynamically adjusted according to the slab widening conditions and the rolling mill status.

[0088] After the steel plate prepared by the above preparation method is welded by submerged arc welding (SAW), the yield strength of the base material of the steel plate is 392MPa, the tensile strength is 530MPa, A is 31%, the average impact energy of the core at -60℃ is ≤80J, and the CEV is 0.38; after automatic welding with the same equipment as in Examples 1 and 2 and a line input of 30kJ / cm, the tensile strength of the joint is 522MPa, the impact energy of the core at -60℃ in the heat-affected zone is ≤30J, the impact toughness is poor, and the weld joint is at a stress ratio of 0.5 and a cycle of 5×10 7The ultimate fatigue stress under these conditions is about 260MPa, which cannot meet the needs of high-energy-density welding.

[0089] The process parameters (such as rolling deformation, holding time, etc.) of the present invention can realize the method by taking upper and lower limits and interval values, and the embodiments are not listed here one by one.

[0090] Any content not described in detail in the present invention can be based on conventional technical knowledge in the art.

[0091] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0092] A thick, fatigue-resistant, and easily weldable high-strength steel plate for wind power and its preparation method. The preparation method is innovative in chemical composition design, billet cooling process, and rolling process, mainly including:

[0093] 1) Low C and high Mn, Nb+V+Ti composite microalloying is adopted to form submicron-sized Ti(C, N) and V(C, N) precipitates, which significantly inhibit the growth of austenite grains and provide a microstructural basis for the subsequent rolling process. The Cottrell and Snoek gas clouds formed by the Nb\Ti(C, N) composite nanocarbides formed in the finishing rolling stage can significantly hinder the movement of dislocations and pin the grain boundaries, inhibiting the growth of austenite grains. At the same time, due to the addition of Nb, V, and Ti microalloying elements, the upper limit of the non-recrystallization temperature is increased, the rolling process range is expanded, and the probability of mixed crystals in the two-phase region is reduced, which is beneficial to improving strength and toughness.

[0094] 2) The slab triple cooling equipment quenching process is adopted to achieve shallow surface quenching of the slab, forming a shell with uniform thickness on the surface, achieving rapid cooling of the slab and defect-free preparation of the slab surface, and then using the self-tempering effect of the slab residual heat to refine the surface grains of the casting.

[0095] 3) The rough rolling adopts a gradient increase mode of reduction rate, and the finishing rolling adopts a gradient decrease mode of reduction rate to fully refine the grains and ensure the rolling penetration effect. Among them, MULPIC cooling of the intermediate billet before finishing rolling can reduce the waiting time of the intermediate billet. At the same time, the normalizing effect can maximize the retention of the dynamic recrystallization grain refinement effect in the rough rolling stage.

[0096] 4) The normalizing rolling process is used to produce thick-gauge steel plates for wind power generation. The steel plates are normalized during the rolling process, eliminating the need for offline normalizing processes. The process flow is short, the cost is low, the production line has strong adaptability, and it has broad promotion prospects.

[0097] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing thick-gauge fatigue-resistant and easily welded high-strength steel plates for wind power, characterized in that: The steps include: Smelting: using converter for smelting; Refining, refining includes LF refining and RH refining, Continuous casting, using a continuous casting mold to produce slabs with a thickness of ≥300mm, After being cut by fire, the slab enters the third cooling machine for water cooling to achieve shallow surface quenching of the slab surface. During the quenching process, the water volume of the lower part is controlled to be greater than the water volume of the upper part. The difference between the upper and lower water volumes is controlled at 15% to 30%. The surface temperature of the slab is controlled to be ≤ 600°C. After the surface γ-α transformation is completed, the surface cooling water is blown away. The slab after continuous casting is subjected to slow cooling treatment. For heating, the slab is heated by cold charging. Rolling, rolling includes rough rolling and finishing rolling. First, rough rolling is carried out. The intermediate billet after rough rolling is cooled by MULPIC water cooler, then transported to finishing mill for finishing rolling, and then enters straightening machine for straightening to obtain finished steel plate. In the rolling step, the rough rolling adopts the large reduction mode. The rough rolling stage is completed by 4+1 passes or 6+1 passes according to the target steel plate thickness, of which the +1 pass is an empty pass. Control the rolling pass reduction rate gradient, the first pass reduction rate ≥ 6%, The reduction rate of the last pass in the 4+1 pass and 6+1 pass rolling modes should be ≥20%, and the pass reduction rate gradient increases by 3% to 8%. In the rolling step, the finish rolling adopts the normalizing rolling mode, the control pass is 6+1 mode, wherein the +1 pass is an empty pass, and the reduction rate decreases gradually. When the thickness of the finished product is 60mm-80mm, the first pass reduction rate is ≥12%, and when the thickness of the finished product is 80mm-90mm, the first pass reduction rate is ≥14%, and the reduction rate decreases by 0.5%-4% with each pass. The steel plate is composed of the following chemical components by mass percentage: C: 0.12%~0.15%, Si: 0.15%~0.35%, Mn: 1.30%~1.50%, P≤0.015%, S≤0.005%, Nb: 0.015%~0.030%, V: 0.015%~0.030%, Ti: 0.005%~0.020%, Al: 0.005%~0.030%, and the rest is Fe and unavoidable impurities.

2. The preparation method according to claim 1, characterized in that The thickness of the steel plate is 70 mm to 90 mm, and the base material yield strength of the steel plate is ≥420 MPa, the tensile strength is ≥540 MPa, A is ≥40%, the core impact energy at -60°C is ≥200 J, and CEV is ≤0.41; The steel plate can be welded under the condition of a large wire input of ≥30 kJ / cm. After welding, the tensile strength of the joint is ≥540 MPa, the impact energy of the core of the heat-affected zone at -60°C is ≥120 J, the stress ratio of the welded joint is 0.5, the cycle is 5×10 7 The ultimate fatigue stress under these conditions is ≥400MPa.

3. The preparation method according to claim 1, characterized in that During the smelting step, the converter adopts a one-time carbon pulling and single slag process for smelting, the final slag basicity R is controlled between 3.2 and 4.2, a red clean ladle is used, the ladle temperature is ≥800°C, the steel discharge time is ≥3 minutes, high-sensitivity slag blocking is used for tapping, 1.7kg / t to 3.2kg / t of aluminum manganese iron is used for deoxidation, low-carbon and low-phosphorus silicon manganese, ferrosilicon, ferroniobium, and ferrovanadium are added in batches, and synthetic slag and pre-melted slag are added along the steel flow during tapping, with the ratio of synthetic slag to pre-melted slag being 2:

1.

4. The preparation method according to claim 1, characterized in that During the LF refining process, argon is blown from the bottom for stirring throughout the process. Aluminum particles and aluminum slag are deoxidized, and lime is added for slagging. The final slag basicity is ≥2.

7. Silicon manganese, ferrosilicon, ferroniobium, and ferrovanadium alloys are used for fine-tuning the composition. Titanium wire and ferrotitanium are used to adjust the Ti composition; aluminum wire is used to adjust the Al composition. The LF refining treatment time is controlled to be ≥40 minutes. After LF refining is completed, RH refining is carried out, and the RH refining time is ≥45min. After the RH refining treatment is completed, 85m~160m of calcium aluminum wire is fed, and then soft blowing is carried out for more than 9 minutes.

5. The preparation method according to claim 1, characterized in that In the continuous casting step, the casting speed of the continuous casting machine is 0.8m / min to 1.1m / min, the peritectic steel protective slag is used, and the crystallizer vibration mode is a non-sinusoidal vibration mode. Secondary cooling water and dynamic soft reduction are controlled according to peritectic steel. The slab is self-tempered using waste heat, and the self-tempering time is ≥120min. After the self-tempering is completed, it is put into the pit for slow cooling, and the slow cooling time is not less than 56 hours.

6. The preparation method according to claim 1, characterized in that In the heating step, the slab charging temperature is ≤350℃, the soaking zone temperature of the heating furnace is controlled to be 1180℃~1260℃, and the tapping temperature is 1170℃~1250℃. The heating rate is 9 min / cm~11.5 min / cm.

7. The preparation method according to claim 6, characterized in that The soaking zone temperature of the heating furnace is 1250°C, the tapping temperature is 1200°C, and the heating rate is 10 min / cm.

8. The preparation method according to claim 1, characterized in that When using 300mm slab to produce 90mm steel plate, the rough rolling passes are 5 passes, the first pass reduction rate is 7%, the last pass reduction rate is 27.5%, and the gradient increments are 5%, 7%, and 8.5% respectively.

9. The preparation method according to claim 1, characterized in that When using 300mm slab to produce 90mm steel plate, 6+1 finishing rolling is adopted, with the first pass reduction rate of 16.5%, and the reduction rate decreases by 1.0% to 3.5% with each pass, and the reduction rate at the 6th pass is 5.8%.

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