A heat treatment process for structural steel used in wind turbine towers

Abstract

This invention discloses a heat treatment process for structural steel used in wind turbine towers, comprising the following steps: heating the steel plate as a whole to 880~920℃, austenitizing and holding at that temperature for 1.0~2.0 min / mm plate thickness; dividing the steel plate into three virtual regions—a surface region, a transition region, and a core region—symmetrically on both sides along the thickness direction, and cooling each region independently; heating the steel plate to 450~550℃, holding for 1~3 hours, and then slowly cooling it in the furnace to below 300℃ before air cooling. This invention employs gradient zone cooling of the steel plate, achieving "one steel for three uses" under the conventional Q345 / Q355 composition system simply by differentiating heat treatment parameters. This avoids the complexity of procuring different grades of steel for different tower sections. Furthermore, the gradient transition region eliminates the interface of abrupt performance changes, and the residual compressive stress field introduced by the rapid cooling of the surface layer significantly improves the hardness, fatigue limit, and elongation after fracture of the steel plate.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for structural steel, and in particular to a heat treatment process for structural steel used in wind turbine towers. Background Technology

[0002] The wind turbine tower (or wind turbine casing) is the core supporting structure of a wind turbine generator set. It bears complex alternating loads such as the weight of the nacelle and rotor, wind loads, and seismic loads. Its design life is typically 20-25 years, and its fatigue life must meet the 2×10⁻⁶ life requirement specified by the IEC 61400 standard. 7 This cycle is used for verification. A typical conical steel tower is 80-120 meters high, with a base diameter of 4.5-6 meters, and the wall thickness gradually decreases from 40-60 mm at the bottom to 20-30 mm at the top. The self-weight of a single onshore wind turbine tower can reach 200-300 tons, and the annual steel consumption exceeds 5 million tons. Currently, the steel used for wind turbine towers mainly adopts grades such as Q355NE (normalized or normalized rolled delivery) and Q420NE (TMCP + microalloyed delivery), and their heat treatment or thermomechanical treatment processes aim to achieve uniformity in the overall microstructure and properties of the steel plate. However, as the capacity of onshore wind turbines rapidly increases from 3MW to 6-8MW or even over 10MW, the load-bearing differences at different heights of the tower are becoming increasingly significant. Steel plates with uniform properties can no longer meet the differentiated requirements for strength, fatigue, toughness, and weldability in different parts. Specifically, the stress characteristics of conical towers exhibit a significant gradient with height: the base section bears the greatest bending moment and self-weight, requiring high strength and high fatigue resistance; the middle section bears moderate bending moment, requiring a balanced match of strength, fatigue, and toughness; the top section experiences the largest wind-induced vibration amplitude, making fatigue resistance and lightweighting requirements most prominent. Existing normalizing processes, such as the low-temperature weldability steel and production method disclosed in patent CN104862589A, apply uniform heating and cooling to the entire cross-section of the steel plate to obtain a homogeneous microstructure, which cannot provide customized gradient performance for different height sections under the same grade. The yield strength of normalized Q355NE steel plates is typically only 355~420MPa, and the fatigue limit is about 250MPa, which is insufficient to meet the high load-bearing requirements of the tower base section of large-capacity units; even if the strength is increased to over 420MPa using TMCP technology, it is still a homogeneous microstructure design, without differentiated treatment from the perspective of structural load-bearing gradient. In conclusion, the existing heat treatment process for structural steel used in wind turbine towers still needs improvement. Summary of the Invention

[0003] To address the above problems, this invention provides a heat treatment process for structural steel used in wind turbine towers, comprising the following steps: S1: Heat the steel plate as a whole to 880~920℃, perform austenitization and hold for a period of time of 1.0~2.0min / mm plate thickness; S2: The steel plate is divided into three virtual regions—surface region, transition region, and core region—in a symmetrical manner along its thickness direction. Each of the three regions is cooled independently. When the steel plate is used in the tower base section and the wall thickness is 40~80mm, the surface area is cooled to 280~320℃ at a cooling rate of 28~35℃ / s and then air-cooled; the transition area is cooled to 420~480℃ at a cooling rate of 8~12℃ / s and then air-cooled; and the core area is cooled to 660~690℃ at a cooling rate of 2~4℃ / s and then air-cooled. When the steel plate is used in the middle section of the tower and the wall thickness is 30~40mm, the surface area is cooled to 280~320℃ at a cooling rate of 25~30℃ / s and then air-cooled; the transition area is cooled to 430~480℃ at a cooling rate of 8~10℃ / s and then air-cooled; and the core area is cooled to 650~680℃ at a cooling rate of 3~5℃ / s and then air-cooled. When the steel plate is used in the top section of the tower and the wall thickness is 20~30mm, the surface area is cooled to 250~300℃ at a cooling rate of 25~30℃ / s and then air-cooled; the transition area is cooled to 400~450℃ at a cooling rate of 6~10℃ / s and then air-cooled; and the core area is cooled to 650~680℃ at a cooling rate of 3~5℃ / s and then air-cooled. S3: Heat the steel plate to 450~550℃, hold for 1~3 hours, and then slowly cool it in the furnace to below 300℃ before air cooling. Furthermore, in step S2, multiple sets of nozzles with independently adjustable cooling intensity are set on the upper and lower surfaces of the steel plate, respectively corresponding to the surface area, transition area and core area, so that the steel plate forms a controlled temperature gradient field along the thickness direction. Furthermore, the surface area is cooled by high-pressure jet water cooling, with a single nozzle water flow rate of 600~900 m³ / h and a water supply pressure of 0.50~0.80 MPa; the transition area is cooled by aerosol cooling, with a single nozzle water flow rate of 150~350 m³ / h, a water supply pressure of 0.20~0.40 MPa, and an air-to-water mass ratio of 0.5:1~2.0:1; the core area is cooled by low-pressure jet water cooling, with a single nozzle water flow rate of 10~60 m³ / h and a water supply pressure of 0.05~0.20 MPa. Furthermore, in step S2, the surface area is from the upper and lower surfaces of the steel plate to 1 / 4 of the plate thickness, the transition area is from 1 / 4 of the plate thickness to 3 / 4 of the plate thickness, and the core area is from 3 / 4 of the plate thickness to the center. Furthermore, the treated steel plate forms a continuous gradient hardness distribution along the thickness direction: surface hardness 240~300HV10, core hardness 170~195HV10, and transition zone hardness between the two. Furthermore, the overall tensile properties of the treated steel plate are as follows: yield strength 420~530MPa, elongation after fracture 24~30%, and longitudinal impact energy at the core of -40℃ 80~120J. Furthermore, the fatigue limit of the treated steel plate reaches 320~380MPa. Compared with the prior art, the beneficial effects of the present invention are: 1. Customized performance to solve gradient load-bearing challenges: The steel plate at the tower base achieves maximum hardness and fatigue resistance through the most intense surface rapid cooling, matching the maximum bending moment requirements at the tower bottom; the steel plate at the tower top moderately reduces the cooling rate and increases the bainite ratio, matching the stringent requirements of the tower top for high fatigue resistance, lightweight, and cold-forming performance; the middle section employs balanced cooling parameters to achieve the optimal balance between strength, plasticity, and toughness. Under the conventional Q345 / Q355 composition system, "one steel for three uses" is achieved solely through differences in heat treatment parameters, avoiding the complexity of procuring different grades of steel for different tower sections. 2. Synergistic breakthrough of high strength, high fatigue, and high toughness and plasticity: The high-hardness martensite / bainite structure on the surface inhibits the initiation of fatigue cracks, the high-plasticity ferrite / pearlite structure in the core hinders crack propagation, the gradient transition zone eliminates the interface of abrupt performance change, and the residual compressive stress field introduced by the rapid cooling of the surface layer increases the fatigue limit of the steel plate by 46~58% compared with the normalized state, while the impact energy at -40℃ is increased by 80~130%. Detailed Implementation The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a specific posture. If the specific posture changes, the directional indication will also change accordingly. Furthermore, the use of terms such as "first" and "second" in this invention is for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. In the description of the embodiments, unless otherwise explicitly specified and limited, the terms "set," "connect," etc., should be interpreted broadly. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or a connection through an intermediate medium, or it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances. The heat treatment process for the structural steel used in wind turbine towers in this invention includes the following steps: S1: Heat the steel plate as a whole to 880~920℃, austenitize it and hold it at that temperature. The holding time is calculated at 1.0~2.0min / mm plate thickness. The steel plate adopts the structural steel grade Q345 or Q355 in GB / T 1591-2018 "Low Alloy High Strength Structural Steel". S2: The steel plate is divided into three virtual regions—surface region, transition region, and core region—in a symmetrical manner along its thickness direction. Each of the three regions is cooled independently. When steel plates are used in the tower base section (wall thickness 40~80mm), the surface area is rapidly cooled to 280~320℃ at a cooling rate of 28~35℃ / s and then air-cooled to obtain a fine-grained martensite + bainite structure; the transition area is cooled at a medium-speed cooling rate of 8~12℃ / s to 420~480℃ and then air-cooled to obtain a fine-grained bainite + a small amount of ferrite structure; the core area is slowly cooled at a cooling rate of 2~4℃ / s to 660~690℃ and then air-cooled to obtain a fine-grained ferrite + pearlite structure. When steel plates are used in the middle section of the tower (wall thickness 30~40mm), the surface area is rapidly cooled to 280~320℃ at a cooling rate of 25~30℃ / s and then air-cooled to obtain a fine-grained bainite + a small amount of martensite structure; the transition area is cooled at a medium speed of 8~10℃ / s to 430~480℃ and then air-cooled to obtain a fine-grained bainite + ferrite structure; the core area is slowly cooled at a cooling rate of 3~5℃ / s to 650~680℃ and then air-cooled to obtain a fine-grained ferrite + pearlite structure. When steel plates are used in the tower top section (wall thickness 20~30mm), the surface area is rapidly cooled to 250~300℃ at a cooling rate of 25~30℃ / s and then air-cooled to obtain a high proportion of fine-grained bainite structure; the transition area is cooled at a medium speed of 6~10℃ / s to 400~450℃ and then air-cooled to obtain a fine-grained bainite + ferrite structure; the core area is slowly cooled at a cooling rate of 3~5℃ / s to 650~680℃ and then air-cooled to obtain a fine-grained ferrite + a small amount of pearlite structure. The surface layer is from the upper and lower surfaces of the steel plate to 1 / 4 of the plate thickness, the transition layer is from 1 / 4 of the plate thickness to 3 / 4 of the plate thickness, and the core layer is from 3 / 4 of the plate thickness to the center. During the actual cooling process, multiple sets of nozzles with independently adjustable cooling intensity are installed on the upper and lower surfaces of the steel plate, corresponding to the surface zone, transition zone, and core zone respectively, creating a controlled temperature gradient field along the thickness direction of the steel plate. Specifically, cooling of the surface zone is achieved through high-pressure jet water cooling, with a single nozzle flow rate of 600~900 m³ / h and a supply pressure of 0.50~0.80 MPa; cooling of the transition zone is achieved through aerosol cooling, with a single nozzle flow rate of 150~350 m³ / h, a supply pressure of 0.20~0.40 MPa, and an air-to-water mass ratio of 0.5:1~2.0:1; cooling of the core zone is achieved through low-pressure jet water cooling, with a single nozzle flow rate of 10~60 m³ / h and a supply pressure of 0.05~0.20 MPa. S3: Heat the cooled steel plate to 450~550℃, hold for 1~3 hours, and then slowly cool it in the furnace to below 300℃ before air cooling to release the trace residual stress generated during the partitioned cooling process, without changing the gradient structure that has been formed. The steel plate after the above heat treatment can achieve the following mechanical properties: (1) A continuous gradient hardness distribution is formed along the thickness direction: the surface hardness is 240~300HV10, the core hardness is 170~195HV10, and the hardness of the transition zone is between the two. (2) The overall tensile properties are: yield strength 420~530MPa, elongation after fracture 24~30%, longitudinal impact energy of the core at -40℃ 80~120J; (3) The fatigue limit reaches 320~380MPa. To more clearly demonstrate the technical effects achievable by the present invention, the implementation process of the present invention will be described in detail below with reference to specific embodiments and comparative examples. Example 1: 50mm thick Q355D steel plate for tower base section 50mm thick Q355D steel plate is selected, and the chemical composition meets the requirements of GB / T 1591-2018 for Q355D. Process: The steel plate is heated to 920℃ and held for 60 minutes; then it is sent to the zoned cooling device and cooled according to the parameters of the tower base section—the surface zone (0~12.5mm) is sprayed with water at a rate of 30℃ / s to 300℃ and then air-cooled; the transition zone (12.5~37.5mm) is cooled with air mist at a rate of 10℃ / s to 450℃ and then air-cooled; the core zone (37.5mm~center) is air-cooled at a rate of 3℃ / s to 680℃ and then air-cooled; finally, it is held at 500℃ for 2 hours and then slowly cooled in the furnace to 280℃ before being taken out of the furnace. The microstructure after treatment is as follows: the surface layer is fine-grained martensite + bainite (grain size grade 11), the transition zone is fine-grained bainite + a small amount of ferrite (grain size grade 10~11), and the core is fine-grained ferrite + pearlite (grain size grade 11). Mechanical properties: Surface hardness 288 HV10, core hardness 180 HV10; tensile properties (full thickness plate specimen) Rel=505 MPa, Rm=615 MPa, A=28.5%; longitudinal impact energy of core at -40℃ KV2=108 J; fatigue limit (R=0.1) σ-1=360 MPa. Example 2: 40mm thick Q355E steel plate in the middle section of the tower 40mm thick Q355E steel plate is selected, and the chemical composition meets the requirements of GB / T 1591-2018 for Q355E. Process: Heat the steel plate to 900℃ and hold for 50 minutes; according to the parameters of the middle section of the tower—the surface zone (0~10mm) is rapidly cooled to 310℃ by air cooling at 28℃ / s, the transition zone (10~30mm) is cooled to 460℃ by air mist cooling at 9℃ / s, and the core zone (30mm~center) is cooled to 670℃ by air cooling at 4℃ / s; finally, it is held at 480℃ for 1.5 hours and slowly cooled to 250℃ before being taken out of the furnace. Mechanical properties: Surface hardness 260 HV10, core hardness 178 HV10; Rel=475 MPa, Rm=585 MPa, A=27.8%; impact energy at -40℃ KV2=95 J; fatigue limit σ-1=335 MPa. Example 3: 25mm thick Q345D steel plate for the top section of the tower 25mm thick Q345D steel plate is selected, and the chemical composition meets the requirements of GB / T 1591-2018 for Q345D. Process: Heat the steel plate to 880℃ and hold for 30 minutes; according to the parameters of the top section of the tower—the surface area (0~6mm) is rapidly cooled to 280℃ by air cooling at 25℃ / s, the transition area (6~19mm) is cooled to 430℃ by air mist cooling at 8℃ / s, and the core area (19mm~center) is cooled to 660℃ by air cooling at 4℃ / s; finally, it is held at 470℃ for 1 hour and slowly cooled to 220℃ before being taken out of the furnace. Mechanical properties: Surface hardness 255 HV10, core hardness 175 HV10; Rel=465 MPa, Rm=575 MPa, A=29.0%; impact energy at -40℃ KV2=98 J; fatigue limit σ-1=350 MPa. Comparative Example 1 (Traditional Poisoning) Heated to 900℃ and held for 1 hour, then air-cooled to room temperature. The resulting material exhibited a uniform ferrite + pearlite microstructure (grain size 8-9) and consistent cross-sectional hardness (175-178 HV10). Mechanical properties: Rel=385 MPa, Rm=510 MPa, A=26.0%; -40℃ impact energy KV2=52 J; fatigue limit σ-1=240 MPa. Comparative Example 2 (Traditional Conditioning) Heated to 900℃ and held for 1 hour, then water quenched to room temperature, followed by tempering at 620℃ for 2 hours and air cooling. The surface hardness after treatment is 210 HV10, and the core hardness is 195 HV10. Mechanical properties: Rel=465 MPa, Rm=570 MPa, A=20.0%; Impact energy at -40℃ KV2=38 J; Fatigue limit σ-1=280 MPa. Comparative Example 3 (Overall Two-Stage Cooling) The sample was heated to 900℃ and held for 1 hour. It was then rapidly cooled to 680℃ by water spraying at 28℃ / s, followed by air cooling to 550℃ at 3℃ / s and then further air-cooled. The treated section exhibited uniform hardness (~190 HV10), and the microstructure consisted of uniform ferrite, bainite, and a small amount of pearlite. Mechanical properties: Rel=410 MPa, Rm=535 MPa, A=25.0%; Impact energy at -40℃ KV2=60 J; Fatigue limit σ-1=260 MPa. The overall performance comparison of the above specific embodiments and comparative examples is shown in the table below: Performance indicators Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Rel(MPa) 505 475 465 385 465 410 Rm(MPa) 615 585 575 510 570 535 A(%) 28.5 27.8 39.0 26.0 20.0 25.0 -40℃ KV2(J) 108 95 98 52 38 60 Fatigue limit σ⁻¹ (MPa) 360 335 350 240 280 260 Surface hardness (HV10) 288 260 255 175 210 190 Heart hardness (HV10) 180 178 175 175 195 185 The above comparative analysis shows that: (1) Compared with Comparative Example 1, the yield strength of Example 1 increased by 31.2%, the fatigue limit increased by 50.0%, and the impact energy at -40℃ increased by 107.7%, achieving a generational improvement in strength, fatigue and toughness, and the elongation increased instead of decreasing. (2) Compared with Comparative Example 2, Example 1, with slightly higher strength, increased elongation and impact energy by 42.5% and 184.2% respectively, and increased fatigue limit by 28.6%, thus completely solving the problem of deterioration of plasticity and toughness of quenched and tempered steel. (3) Compared with Comparative Example 3, Example 1 has significantly higher strength and fatigue limit, and the impact energy is increased by 80%, proving that the "external strength and internal toughness" synergistic effect of the present invention cannot be achieved by relying solely on overall segmented cooling. (4) Compared with Comparative Examples 1 to 3, Examples 1 to 3, under conventional Q345 / Q355 composition, have fatigue limits that are 50.0%, 19.6% and 34.6% higher, respectively, and have superior elongation, strength and other process properties. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them; when the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope claimed by the present invention.

Claims

1. A heat treatment process for a structural steel for wind towers, characterized by, Includes the following steps: S1: Heat the steel plate as a whole to 880~920℃, perform austenitization and hold for a period of time of 1.0~2.0min / mm plate thickness; S2: The steel plate is divided into three virtual regions—surface region, transition region, and core region—in a symmetrical manner along its thickness direction. Each of the three regions is cooled independently. When the steel plate is used in the tower base section and the wall thickness is 40~80mm, the surface area is cooled to 280~320℃ at a cooling rate of 28~35℃ / s and then air-cooled; the transition area is cooled to 420~480℃ at a cooling rate of 8~12℃ / s and then air-cooled; and the core area is cooled to 660~690℃ at a cooling rate of 2~4℃ / s and then air-cooled. When the steel plate is used in the middle section of the tower and the wall thickness is 30~40mm, the surface area is cooled to 280~320℃ at a cooling rate of 25~30℃ / s and then air-cooled; the transition area is cooled to 430~480℃ at a cooling rate of 8~10℃ / s and then air-cooled; and the core area is cooled to 650~680℃ at a cooling rate of 3~5℃ / s and then air-cooled. When the steel plate is used in the top section of the tower and the wall thickness is 20~30mm, the surface area is cooled to 250~300℃ at a cooling rate of 25~30℃ / s and then air-cooled; the transition area is cooled to 400~450℃ at a cooling rate of 6~10℃ / s and then air-cooled; and the core area is cooled to 650~680℃ at a cooling rate of 3~5℃ / s and then air-cooled. S3: Heat the steel plate to 450~550℃, hold for 1~3 hours, and then slowly cool it in the furnace to below 300℃ before air cooling.

2. The process for heat treatment of structural steel for wind towers according to claim 1, characterized by the fact that: In step S2, multiple sets of nozzles with independently adjustable cooling intensity are set on the upper and lower surfaces of the steel plate, corresponding to the surface area, transition area and core area respectively, so that a controlled temperature gradient field is formed in the thickness direction of the steel plate.

3. The process for heat treatment of structural steel for wind towers according to claim 2, characterized by the fact that: The surface area is cooled by high-pressure jet water cooling, with a single nozzle water flow rate of 600~900 m³ / h and a water supply pressure of 0.50~0.80 MPa; the transition area is cooled by aerosol cooling, with a single nozzle water flow rate of 150~350 m³ / h, a water supply pressure of 0.20~0.40 MPa, and an air-to-water mass ratio of 0.5:1~2.0:1; the core area is cooled by low-pressure jet water cooling, with a single nozzle water flow rate of 10~60 m³ / h and a water supply pressure of 0.05~0.20 MPa.

4. The process for heat treatment of structural steel for wind towers according to claim 1, characterized by the fact that: In step S2, the surface area is from the upper and lower surfaces of the steel plate to 1 / 4 of the plate thickness, the transition area is from 1 / 4 of the plate thickness to 3 / 4 of the plate thickness, and the core area is from 3 / 4 of the plate thickness to the center.

5. The process for heat treatment of structural steel for wind towers according to any of claims 1 to 4, characterized by the fact that: After processing, the steel plate forms a continuous gradient hardness distribution along the thickness direction: the surface hardness is 240~300HV10, the core hardness is 170~195HV10, and the transition zone hardness is between the two.

6. The process for heat treatment of structural steel for wind towers according to any of claims 1 to 4, characterized by that: The overall tensile properties of the treated steel plate are as follows: yield strength 420~530MPa, elongation after fracture 24~30%, and longitudinal impact energy of the core at -40℃ 80~120J.

7. The process for heat treatment of structural steel for wind towers according to any of claims 1 to 4, characterized by that: After treatment, the fatigue limit of the steel plate reaches 320~380MPa.