Normalized steel plate for wind power tower door frame and preparation method of normalized steel plate

Abstract

The invention relates to a normalized steel plate for a wind power tower door frame and a preparation method thereof, and belongs to the technical field of steel materials. The normalized steel plate comprises the following chemical components in percentage by mass: 0.15%-0.18% of C, 0.20%-0.50% of Si, 1.60%-1.80% of Mn, 0.02%-0.03% of Nb, 0.025%-0.045% of V, 0.0050%-0.0080% of N, less than or equal to 0.010% of P, less than or equal to 0.003% of S, 0.44%-0.46% of carbon equivalent, less than or equal to 0.010% of Alt and the balance of Fe. Wherein 6 < = (V + Nb) / N < = 10, and in the formula, (V + Nb) / N represents the ratio of the sum of the mass fractions of V and Nb to the mass fraction of N. A middle-high C and high Mn design is adopted, C and Mn solid solution strengthening and Nb and V composite microalloying precipitation strengthening are utilized, the (V + Nb) / N ratio is optimized, free C and free N are reduced, and aging deterioration is avoided. Grain is refined and a precipitated phase is optimized through processes such as low-speed large-pressure swing cooling, internal stress is eliminated by combining second stack cooling and normalizing heat treatment, the microstructure is improved, and the purpose that the normalized steel plate has the comprehensive performance of high strength, good low-temperature toughness and low aging sensitivity is achieved.

Description

Technical Field

[0001] The present application relates to the technical field of steel materials, and in particular to a normalized steel plate for a wind power tower door frame and a preparation method thereof. Background Art

[0002] The wind turbine tower is the tower pole of wind power generation. It mainly plays a supporting role in the wind turbine generator set. With the increase in the capacity of wind turbines, the strength level of steel plates has been increased from the traditional 355MPa to 420MPa. A door opening needs to be opened at the bottom of the wind turbine tower, and a corresponding door frame is installed on the door opening as a passage for staff to enter the tower. Therefore, the door frame of the wind turbine tower is an important part of the wind turbine tower. The plate material of the door frame of the wind turbine tower is generally made of thicker steel plates. The steel plates need to be pressed multiple times to produce a door frame shape that meets the design requirements. In addition, in order to eliminate the stress during the cold processing deformation process, the door frame needs to be normalized to eliminate stress.

[0003] The door frame not only has to bear the weight of the equipment itself and the load generated by the wind force directly acting on the equipment, but also has a large stress concentration at this location, so the service conditions are harsh. In combination with the processing and manufacturing process of the above-mentioned wind turbine door frame, it is determined that the steel plate for the wind turbine door frame should have the following properties: the steel plate is subjected to normalizing heat treatment, the yield strength is ≥420MPa, the tensile strength is ≥520MPa, the elongation after fracture is ≥19%, the impact energy at -40℃ is ≥100J, and the aging sensitivity coefficient after 5% strain should be within 25%. Therefore, the normalized steel plate for the wind turbine tower door frame must have comprehensive properties such as high strength, good low-temperature toughness and low aging sensitivity, which cannot be met by the existing technology. Summary of the invention

[0004] The present application provides a normalized steel plate for a wind turbine tower door frame and a preparation method thereof, in order to solve the following technical problem: how to make the normalized steel plate for a wind turbine tower door frame have high strength, good low-temperature toughness and low aging sensitivity.

[0005] In a first aspect, the present application provides a normalized steel plate for a wind power tower door frame, wherein the chemical composition of the normalized steel plate comprises, by mass fraction: C: 0.15% to 0.18%, Si: 0.20% to 0.50%, Mn: 1.60% to 1.80%, Nb: 0.02% to 0.03%, V: 0.025% to 0.045%, N: 0.0050% to 0.0080%, P≤0.010%, S≤0.003%, carbon equivalent: 0.44% to 0.46%, Alt≤0.010%, and Fe;

[0006] Among them, 6≤(V+Nb) / N≤10, where (V+Nb) / N represents the ratio of the sum of the mass fractions of V and Nb to the mass fraction of N.

[0007] Optionally, the microstructure of the normalized steel plate is ferrite and pearlite.

[0008] Optionally, the grain size of the ferrite is 7.5 μm to 11.5 μm.

[0009] Optionally, the normalized steel plate satisfies at least one of the following properties: yield strength>430MPa, tensile strength>530MPa, -40°C impact energy single value>130J, -40°C strain aging impact energy single value>100J.

[0010] In a second aspect, the present application provides a method for preparing the normalized steel plate for the wind power tower door frame described in the first aspect, the method comprising:

[0011] Obtaining molten steel having the chemical composition;

[0012] Continuously casting the molten steel to obtain a continuously cast billet;

[0013] The continuous casting billet is sequentially subjected to first batch cooling, heating, rough rolling and finish rolling to obtain a semi-finished steel plate;

[0014] performing a first cooling on the semi-finished steel plate;

[0015] The semi-finished steel plate after the first cooling is sequentially subjected to second stack cooling, normalizing heat treatment and second cooling to obtain a finished steel plate.

[0016] Optionally, the thickness of the continuous casting billet is 300 mm to 400 mm.

[0017] Optionally, the temperature of the first cooling is 400° C. to 700° C., and the time of the first cooling is >36 hours.

[0018] Optionally, the heating temperature is 1140° C. to 1170° C., and the heating time is ≥ 240 min.

[0019] Optionally, the rough rolling is low-speed high-reduction rolling, the rolling speed of the rough rolling is 0.5m / s to 1.0m / s, the single-pass reduction of the rough rolling is ≥35mm, and the intermediate warm-keeping thickness of the rough rolling is 2 to 3 times the thickness of the finished steel plate.

[0020] Optionally, the start rolling temperature of the finishing rolling is 870°C to 850°C, and the final rolling temperature of the finishing rolling is 840°C to 850°C.

[0021] Optionally, the continuous casting billet is subjected to first batch cooling, heating, rough rolling and finish rolling in sequence to obtain a semi-finished steel plate;

[0022] The continuously cast billet is subjected to first pile cooling, heating, dephosphorization, rough rolling and finish rolling in sequence to obtain a semi-finished steel plate; wherein the rough rolling is a multi-pass rough rolling, and in the interval between two adjacent passes of the rough rolling, the dephosphorization water is used to cool the continuously cast billet.

[0023] Optionally, the first cooling is swing cooling, the start cooling temperature of the first cooling is 790°C to 820°C, the final cooling temperature of the first cooling is 350°C to 400°C, and the cooling rate of the first cooling is 15°C / s to 25°C / s.

[0024] Optionally, the temperature of the second cooling stack is >300° C., and the stacking time of the second cooling stack is >36 h.

[0025] Optionally, the heating temperature of the normalizing heat treatment is (Ac3+5)°C to (Ac3+10)°C, the temperature rise coefficient of the normalizing heat treatment is 2.3min / mm to 2.5min / mm, and the holding time of the normalizing heat treatment is 5min to 10min.

[0026] Optionally, the second cooling is to naturally cool the semi-finished steel plate to 550° C. to 580° C., and then to stack cool the semi-finished steel plate to room temperature.

[0027] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0028] The present application provides a normalized steel plate for a wind power tower door frame. The chemical composition of the normalized steel plate includes, by mass fraction: C: 0.15% to 0.18%, Si: 0.20% to 0.50%, Mn: 1.60% to 1.80%, Nb: 0.02% to 0.03%, V: 0.025% to 0.045%, N: 0.0050% to 0.0080%, P≤0.010%, S≤0.003%, carbon equivalent: 0.44% to 0.46%, Alt≤0.010%, and Fe; wherein 6≤(V+Nb) / N≤10, wherein (V+Nb) / N represents the ratio of the sum of the mass fractions of V and Nb to the mass fraction of N. The basic composition design of medium-high carbon and high manganese is adopted, and the solid solution strengthening of C and Mn is used to improve the strength of the steel plate after normalizing. At the same time, Nb and V are added to form a composite microalloying system. These microalloying elements combine with carbon and nitrogen elements in the steel to form carbonitride precipitation phases, and the strength of the steel plate is further improved through the precipitation strengthening mechanism. The ratio of (V+Nb) / N is optimized to reduce the free carbon and nitrogen elements in the steel, avoiding the deterioration of the strain aging performance. At the same time, the normalizing heat treatment also helps to eliminate or reduce the aging sensitivity. By adopting rolling and cooling methods such as low-speed large reduction process and swing cooling process, the grains are refined and the morphology and distribution of the precipitated phase are optimized, thereby improving the low-temperature toughness and impact toughness of the steel plate. In addition, the second pile cooling and normalizing heat treatment also help to further eliminate internal stress and improve the microstructure, thereby improving the low-temperature toughness of the steel plate. In summary, this application achieves the goal of the normalized steel plate having the comprehensive performance of high strength, good low-temperature toughness and low aging sensitivity through carefully designed chemical composition and preparation method. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0030] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0031] Figure 1 A schematic flow chart of a method for preparing a normalized steel plate for a wind power tower door frame provided in an embodiment of the present application;

[0032] Figure 2 The metallographic structure diagram of the steel plate provided in Example 2 of the present application;

[0033] Figure 3 This is the precipitation phase diagram of the steel plate provided in Example 2 of the present application. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0035] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range; in addition, whenever a numerical range is indicated in this document, it is meant to include any cited numbers (fractions or integers) within the indicated range.

[0036] In this document, the terms including "including" and "including" mean "including but not limited to". Relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. "And / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "plurality" means two or more; "at least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items; for example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, if the weight ratio of substance A, substance B and substance C is 1:2:3, then substance A, substance B and substance C should correspond to the proportional numbers in the proportional formula in the order of description, that is, the weight of substance A: the weight of substance B: the weight of substance C = 1:2:3.

[0037] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0038] In a first aspect, the present application provides a normalized steel plate for a wind power tower door frame, wherein the chemical composition of the normalized steel plate comprises, by mass fraction: C: 0.15% to 0.18%, Si: 0.20% to 0.50%, Mn: 1.60% to 1.80%, Nb: 0.02% to 0.03%, V: 0.025% to 0.045%, N: 0.0050% to 0.0080%, P≤0.010%, S≤0.003%, carbon equivalent: 0.44% to 0.46%, Alt≤0.010%, and Fe;

[0039] Among them, 6≤(V+Nb) / N≤10, where (V+Nb) / N represents the ratio of the sum of the mass fractions of V and Nb to the mass fraction of N.

[0040] The positive effect of limiting the mass fraction of C to 0.15% to 0.18% is that it not only ensures that the steel plate has a certain solid solution strengthening effect, but also enables the structure to obtain a pearlite content of 15% to 20%, ensuring the high strength and good low temperature toughness and strain aging impact properties of the steel plate. For example, the mass fraction of C can be 0.15%, 0.155%, 0.16%, 0.165%, 0.17%, 0.175%, 0.18%, etc.

[0041] Positive effects of limiting the mass fraction of Si to 0.20% to 0.50%: Si, as an essential element for steelmaking deoxidation, strongly inhibits and delays the decomposition of carbides of supercooled austenite, improves the stability of austenite, promotes the phase transformation of acicular ferrite, and improves the strength of steel plates. However, the mass fraction of Si has an impact on the low-temperature toughness of steel plates, so the mass fraction of Si is limited to 0.20% to 0.50%. For example, the mass fraction of Si can be 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, etc.

[0042] The positive effect of limiting the mass fraction of Mn to 1.60% to 1.80% is that the cost of Mn alloy is low, and it can increase the toughness and hardness of steel. It is a strong element that stabilizes austenite. The solid solution strengthening effect of Mn can ensure the strength of the steel plate, but it is subject to the carbon equivalent limit of the steel plate, so the mass fraction of Mn is limited to 1.60% to 1.80%. For example, the mass fraction of Mn can be 1.60%, 1.65%, 1.70%, 1.75%, 1.80%, etc.

[0043] The positive effects of limiting the mass fraction of Nb to 0.02% to 0.03% are: on the one hand, the recrystallization of the steel plate is suppressed during the rolling stage, the phase transformation driving force of the steel plate is increased, and the structure is refined, thereby improving the strength and toughness of the steel plate; on the other hand, Nb can form second phase particles with C and N to produce precipitation strengthening. For example, the mass fraction of Nb can be 0.02%, 0.022%, 0.024%, 0.026%, 0.028%, 0.03%, etc.

[0044] The positive effect of limiting the mass fraction of V to 0.025% to 0.045% is that V is one of the microalloying elements in steel, which can refine the grains of steel, thereby improving the strength and toughness of steel. In the range of mass fraction of 0.025% to 0.045%, the effect of V is particularly significant. For example, the mass fraction of V can be 0.025%, 0.030%, 0.035%, 0.040%, 0.045%, etc.

[0045] Positive effects of limiting Alt≤0.010%: Al is an effective deoxidizer. The residual Al in the steel can refine the grain size of the steel plate. It is a relatively cheap fine-grained element. However, due to the addition of a certain amount of nitrogen to the steel, the precipitation temperature of aluminum nitride is high and it will deteriorate the high-temperature thermoplasticity of the steel plate, which can easily lead to cracks on the surface of the ingot. In order to ensure the surface quality of high-nitrogen steel, Alt≤0.010%. For example, the mass fraction of Alt can be 0.002%, 0.004%, 0.006%, 0.008%, 0.010%, etc.

[0046] The positive effect of limiting the mass fraction of N to 0.0050% to 0.0080% is that N can effectively promote the precipitation of Nb and V, and the precipitation size is relatively small, has a good precipitation strengthening effect and does not have an adverse effect on the low-temperature toughness of the steel plate. The mass fraction of nitrogen higher than 0.0080% will significantly deteriorate the strain aging impact toughness of the steel plate. For example, the mass fraction of N can be 0.0050%, 0.0055%, 0.0060%, 0.0065%, 0.0070%, 0.0075%, 0.0080%, etc.

[0047] The positive effect of limiting P≤0.010%, S≤0.003%: P and S are harmful elements in steel, which have adverse effects on the plasticity, low-temperature toughness and weldability of the steel plate. Therefore, in order to ensure the comprehensive mechanical properties of the steel plate, the mass fraction of P and S in the steel plate should be strictly controlled.

[0048] Positive effect of limiting 6≤(V+Nb) / N≤10: By limiting (V+Nb) / N, the excess free carbon and nitrogen elements in the steel can be reduced to avoid deterioration of the strain aging properties of the steel plate. Exemplary values ​​of (V+Nb) / N can be 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, etc.

[0049] The positive effect of limiting the carbon equivalent to 0.44% to 0.46% is as follows: In the fields of metallurgy, metal material science and engineering, carbon equivalent is mainly used to evaluate the welding performance and graphitization tendency of steel and cast iron. It converts the influence of various alloying elements (such as silicon, manganese, phosphorus, sulfur, chromium, molybdenum, vanadium, nickel, copper, etc.) in steel on the material properties according to the degree to which their effects are similar to carbon, thereby obtaining a value equivalent to the carbon content. During the heat treatment process, the carbon equivalent directly affects the organizational transformation and performance changes of the steel. A carbon equivalent of 0.44% to 0.46% is conducive to obtaining a uniform organization and good mechanical properties, thereby ensuring that the finished steel plate can meet the expected performance requirements after normalizing heat treatment. Exemplarily, the carbon equivalent can be 0.44%, 0.444%, 0.448%, 0.452%, 0.456%, 0.46%, etc.

[0050] The specific content / content range of Fe can be obtained by the upper and lower limit formula of the component, namely:

[0051] The sum of the percentages of the contents of each component in a composition should be equal to 100%, and the content ranges of several components should meet the following conditions: the upper limit value of a component + the lower limit value of other components ≤ 100; the lower limit value of a component + the upper limit value of other components ≥ 100.

[0052] In some embodiments, the microstructure of the normalized steel plate is ferrite and pearlite, and the grain size of the ferrite is 7.5 μm to 11.5 μm.

[0053] This microstructure gives the steel plate excellent mechanical properties and toughness. For example, the grain size of ferrite can be 7.5 μm, 8.5 μm, 9.5 μm, 10.5 μm, 11.5 μm, etc.

[0054] In some embodiments, the normalized steel sheet contains Nb—V composite precipitates and a large amount of V(C,N) precipitates with a size of 10 nm to 20 nm.

[0055] In some embodiments, the normalized steel plate satisfies at least one of the following properties: yield strength>430MPa, tensile strength>530MPa, -40°C impact energy single value>130J, -40°C strain aging impact energy single value>100J.

[0056] These performance indicators ensure that the steel plates have sufficient strength and toughness in application scenarios such as wind turbine tower door frames.

[0057] Figure 1 A schematic flow chart of a method for preparing a normalized steel plate for a wind turbine tower door frame provided in an embodiment of the present application.

[0058] See also Figure 1 In a second aspect, the present application provides a method for preparing the normalized steel plate for the wind power tower door frame described in the first aspect, the method comprising:

[0059] S1, obtaining molten steel having the chemical composition;

[0060] S2, continuously casting the molten steel to obtain a continuously cast billet;

[0061] In some embodiments, the thickness of the continuous casting billet is 300 mm to 400 mm.

[0062] The thickness of the continuous casting billet directly affects the mechanical properties of the final product. Within the range of 300mm to 400mm, the continuous casting billet can undergo subsequent heat treatment and rolling processes to obtain good strength and toughness, meeting the high requirements of the wind tower door frame for material properties. If the target thickness is 45mm to 60mm for the steel plate, a 300mm continuous casting billet can be selected. If the target thickness is 60mm to 120mm for the steel plate, a 400mm continuous casting billet can be selected. For example, the thickness of the continuous casting billet can be 300mm, 320mm, 340mm, 360mm, 380mm, 400mm, etc.

[0063] S3, sequentially performing first batch cooling, heating, rough rolling and finish rolling on the continuously cast billet to obtain a semi-finished steel plate;

[0064] In some embodiments, the temperature of the first stack cooling is 400° C. to 700° C., and the time of the first stack cooling is >36 hours.

[0065] Heap of cold refers to a process in which the steel ingots or steel materials after demoulding are piled in a dry and windproof place and cooled naturally in the air. This method slows down the cooling rate of the rolled material by reducing the contact area between the steel and the air. The heap cooling process does not require special cooling equipment, and only requires the steel ingots or steel materials to be piled in a suitable position, so it is easy to operate and has low cost. During the continuous casting process, due to rapid cooling and solidification, certain residual stresses will be generated inside the continuous casting billet. The first heap cooling time is >36h, which can gradually dissipate these stresses and avoid cracking or deformation due to stress concentration during subsequent processing. Exemplarily, the first heap cooling time can be 36.5h, 37h, 38h, 39h, 40h, etc.

[0066] In some embodiments, the heating temperature is 1140° C. to 1170° C., and the heating time is ≥ 240 min.

[0067] The heating temperature is 1140℃~1170℃, which is based on the chemical composition and microstructure transformation of the steel plate to ensure that the steel plate obtains a uniform austenite structure during the heating process. Exemplarily, the heating temperature can be 1140℃, 1145℃, 1150℃, 1155℃, 1160℃, 1165℃, 1170℃, etc. Limiting the heating time to ≥240min can ensure that the steel plate is fully heated and the internal temperature gradient is eliminated. Exemplarily, the heating time can be 240min, 245min, 250min, 255min, 260min, 265min, etc.

[0068] In some embodiments, the rough rolling is low-speed high-reduction rolling, the rolling speed of the rough rolling is 0.5m / s to 1.0m / s, the single-pass reduction of the rough rolling is ≥35mm, and the intermediate warm-keeping thickness of the rough rolling is 2 to 3 times the thickness of the finished steel plate.

[0069] The low-speed large reduction process deforms the core of the extra-thick steel plate by reducing the rolling speed and applying a large reduction, so that the internal defects can be fully bridged, thereby effectively improving the performance of the extra-thick steel plate and reducing various surface quality defects. Limiting the rolling speed of rough rolling to 0.5m / s~1.0m / s helps to reduce the deformation resistance during the rolling process of the steel plate and improve the plasticity of the material. Exemplarily, the rolling speed of rough rolling is 0.5m / s, 0.6m / s, 0.7m / s, 0.8m / s, 0.9m / s, 1.0m / s, etc. Limiting the single-pass reduction of rough rolling to ≥35mm can well improve the deformation permeability of each pass in the rolling process, strengthen the work hardening effect caused by plastic deformation, and improve the strength and hardness of the steel plate. Exemplarily, the single-pass reduction of rough rolling can be 35mm, 36mm, 37mm, 38mm, 39mm, 40mm, etc. The combination of low speed and high reduction helps to eliminate the quality defects of steel plates such as internal cracks, looseness, and pores originating from the steelmaking process. It improves the internal quality and surface quality of the steel plate. It helps to reduce heat loss and deformation resistance during rolling, while promoting the refinement of the microstructure of the steel. The intermediate temperature waiting thickness is limited to 2 to 3 times the thickness of the finished steel plate, in order to provide sufficient phase transformation driving force for subsequent fine grain strengthening.

[0070] In some embodiments, the start rolling temperature of the finish rolling is 850°C to 870°C, and the final rolling temperature of the finish rolling is 840°C to 850°C.

[0071] In order to avoid large-scale precipitation of V during rolling and to avoid rolling in a temperature range where V precipitates in austenite, the start rolling temperature of the finishing rolling is set to 850° C. to 870° C. For example, the start rolling temperature of the finishing rolling may be 850° C., 854° C., 858° C., 862° C., 866° C., 870° C., etc.

[0072] In some embodiments, the continuous casting billet is subjected to first batch cooling, heating, rough rolling and finish rolling in sequence to obtain a semi-finished steel plate;

[0073] The continuously cast billet is subjected to first pile cooling, heating, dephosphorization, rough rolling and finish rolling in sequence to obtain a semi-finished steel plate; wherein the rough rolling is a multi-pass rough rolling, and in the interval between two adjacent passes of the rough rolling, the dephosphorization water is used to cool the continuously cast billet.

[0074] After the continuous casting billet is heated, the continuous casting billet can be dephosphorized to remove the iron oxide scale and impurities on the surface and improve the surface quality of the subsequent rolling process. After dephosphorization, the continuous casting billet quickly enters the rough rolling mill for low-speed large-reduction deformation to promote recrystallization in the rough rolling stage. In order to ensure the deformation of the core and reduce the performance difference of the finished steel plate in the thickness direction, the dephosphorized water is used to cool the continuous casting billet in the gap between the two adjacent rough rolling passes.

[0075] S4, performing a first cooling on the semi-finished steel plate;

[0076] In some embodiments, the first cooling is swing cooling, the start temperature of the first cooling is 790°C to 820°C, the final temperature of the first cooling is 350°C to 400°C, and the cooling rate of the first cooling is 15°C / s to 25°C / s.

[0077] In the embodiment of the present application, the steel plate after final rolling is put into ACC for swing cooling to enhance the water cooling capacity, which can inhibit the large-sized precipitation of V after phase transformation. Putting the steel plate after final rolling into ACC (Accelerated Controlled Cooling) for swing cooling is an advanced steel plate cooling process, which is mainly used to improve the comprehensive mechanical properties of the steel plate.

[0078] S5, performing a second stack cooling, a normalizing heat treatment and a second cooling in sequence on the semi-finished steel plate after the first cooling to obtain a finished steel plate.

[0079] In some embodiments, the temperature of the second cooling stack is >300° C., and the stacking time of the second cooling stack is >36 h.

[0080] The second cooling can promote the overflow of hydrogen in the steel to ensure the internal quality of the steel plate, and can promote the precipitation of V ferrite. For example, the temperature of the second cooling can be 302°C, 312°C, 322°C, 332°C, 342°C, etc. The stacking time of the second cooling is >36h, which can promote the precipitation strengthening effect of the steel plate. For example, the stacking time of the second cooling can be 36.5h, 37h, 38h, 39h, 40h, etc.

[0081] In some embodiments, the heating temperature of the normalizing heat treatment is (Ac3+5)°C to (Ac3+10)°C, the temperature rise coefficient of the normalizing heat treatment is 2.3min / mm to 2.5min / mm, and the holding time of the normalizing heat treatment is 5min to 10min.

[0082] Normalizing is an important metal heat treatment process, which involves heating the steel to a certain appropriate temperature, keeping it warm for a certain period of time, and then slowly cooling it in the air. Normalizing can eliminate the structural defects produced during the hot rolling process, refine the grains, and improve the comprehensive performance of the steel. In this embodiment, on the basis of ensuring that the steel plate is completely austenitized, a low heating temperature is used as much as possible to avoid the growth of austenite. Ac3 is an important critical point in the heat treatment of steel materials, representing the final temperature of austenite formation in the steel during the heating process. Specifically, when the steel is heated to the Ac3 temperature, the pearlite (composed of ferrite and cementite) inside it will be completely transformed into austenite.

[0083] The heating temperature of the normalizing heat treatment is (Ac3+5)℃~(Ac3+10)℃: During the normalizing heat treatment, the steel is heated to 5℃~10℃ above the critical point Ac3 and maintained for a sufficient time to ensure that the required transformation of the internal structure of the steel occurs. Subsequently, the steel is evenly cooled in the free-flowing air to obtain the desired microstructure and properties. This method of precisely controlling the heating temperature helps to refine the grains of the steel, optimize its microstructure, and improve the mechanical properties and processing properties of the steel. At the same time, normalizing heat treatment can also eliminate residual stress in the steel, reduce the tendency to deformation and cracking, and further improve its service life and reliability.

[0084] The positive effect of limiting the temperature rise coefficient of normalizing heat treatment to 2.3min / mm~2.5min / mm: The temperature rise coefficient of 2.3min / mm~2.5min / mm means that it takes 2.3~2.5 minutes for each millimeter of steel thickness to heat to the set temperature of normalizing treatment. The setting of this coefficient is intended to ensure that the steel can be heated to the required temperature evenly and quickly, so as to achieve ideal microstructural transformation and mechanical properties. Exemplarily, the temperature rise coefficient of normalizing heat treatment can be 2.3min / mm, 2.35min / mm, 2.4min / mm, 2.45min / mm, 2.5min / mm, etc.

[0085] The positive effect of limiting the holding time of normalizing heat treatment to 5min to 10min: During the holding process, a series of changes will occur in the structure inside the steel, such as homogenization of austenite, dissolution and precipitation of carbides, etc. These changes require sufficient time to complete to ensure that the steel obtains the required microstructure and mechanical properties. If the holding time is less than 5min, the internal structure transformation of the steel may not be sufficient, resulting in poor performance; on the contrary, if the holding time is greater than 10min, it may cause excessive grain growth of the steel, which will also affect its mechanical properties. Therefore, the holding time of normalizing heat treatment is limited to 10min. Exemplarily, the holding time of normalizing heat treatment can be 5min, 6min, 7min, 8min, 9min, 10min, etc.

[0086] In some embodiments, the second cooling is to naturally cool the semi-finished steel plate to 550° C. to 580° C., and then to stack cool the semi-finished steel plate to room temperature.

[0087] After leaving the heating furnace, the semi-finished steel plate is naturally cooled in the air to 550°C to 580°C to undergo a complete phase change, and then the steel plate is pile-cooled to room temperature to promote the precipitation of V and ensure the strength after normalizing. In the embodiment of the present application, the temperature range of room temperature is 20°C to 100°C.

[0088] In some embodiments, the steel can be used not only to manufacture wind turbine tower door frames, but also to manufacture other structural parts that require high strength, high toughness and good weldability.

[0089] The present application is further described below in conjunction with specific embodiments. The experimental methods in the following embodiments that do not specify specific conditions are usually measured in accordance with national standards / industry standards; if there are no corresponding national standards / industry standards, they are measured in accordance with common international standards, conventional conditions, or conditions recommended by the manufacturer.

[0090] Example 1

[0091] Normalized steel plate for wind power tower door frame with a thickness of 60mm. The designed mass percentage of the steel composition is as follows: C: 0.16%, Si: 0.25%, Mn: 1.65%, V: 0.040%, Nb: 0.025%, Al: 0.007%, N: 0.0073%, and the balance is Fe and unavoidable impurities.

[0092] The specific steps and parameters of the above steel plate production method are as follows:

[0093] 1. The continuous casting is 300mm thick continuous casting billet, and the continuous casting billet is pile-cooled for 36 hours.

[0094] 2. Before the continuous casting billet enters the heating furnace, thick-gauge hot-rolled ordinary plates should be arranged before and after it to ensure the self-adaptation of the rolling mill and the stacking effect and self-tempering of the steel plate.

[0095] 3. The continuous casting billet is heated in a stepping beam heating furnace, the furnace time is 269 minutes, and the billet temperature out of the furnace is 1148℃. Then a two-stage rolling process is adopted. The rough rolling adopts a low-speed large reduction process. The rolling force and rolling torque are set to the maximum allowable capacity of the rolling mill. After five passes, the intermediate waiting thickness is 120mm, and the surface is cooled with dephosphorized water in each pass during the rolling process; the intermediate billet is air-cooled on the roller table, and the non-recrystallization zone rolling begins after cooling to a surface temperature of 860℃. The rolling still adopts a large reduction strategy, rolling to a target thickness of 60mm, and the final rolling surface temperature is 845℃.

[0096] 4. After rolling, the steel plate enters UFC+ACC for swing cooling, and the final cooling temperature is 380℃. Then it is quickly taken off the line for stack cooling for more than 36 hours.

[0097] 5. Heat the steel plate to 865°C in a heat treatment furnace. Keep warm for 10 minutes, then cool it naturally to 550°C in the air. Then cool the steel plate to room temperature.

[0098] After destacking, the whole board was tested for flaw detection, and the flaw detection performance was good. The tensile, impact performance, cold bending, Z-direction performance and strain aging performance tests were carried out on the 120mm surface, 1 / 4 position and 1 / 2 position. The specific performance is shown in Table 1.

[0099] Table 1 Steel plate performance data

[0100]

[0101]

[0102] Example 2

[0103] Normalized steel plate for wind power tower door frame with a thickness of 120mm. The designed mass percentage of the steel composition is as follows: C: 0.16%, Si: 0.28%, Mn: 1.69%, V: 0.043%, Nb: 0.026%, Al: 0.007%, N: 0.0078%, and the balance is Fe and unavoidable impurities.

[0104] The specific steps and parameters of the above steel plate production method are as follows:

[0105] 1. The continuous casting is 400mm thick continuous casting billet, and the continuous casting billet is pile-cooled for 72 hours.

[0106] 2. Before the continuous casting billet enters the heating furnace, thick-gauge hot-rolled ordinary plates should be arranged before and after it to ensure the self-adaptation of the rolling mill and the stacking effect and self-tempering of the steel plate.

[0107] 3. The continuous casting billet is heated in a stepping beam heating furnace, the furnace time is 339 minutes, and the billet temperature out of the furnace is 1152℃. Then a two-stage rolling process is adopted. The rough rolling adopts a low-speed large reduction process. The rolling force and rolling torque are set to the maximum allowable capacity of the rolling mill. After six passes, the intermediate waiting thickness is 180mm, and the surface is cooled with dephosphorized water in each pass during the rolling process; the intermediate billet is air-cooled on the roller table, and the non-recrystallization zone rolling begins after cooling to a surface temperature of 850℃. The rolling still adopts a large reduction strategy, rolling to a target thickness of 120mm, and the final rolling surface temperature is 842℃.

[0108] 4. After rolling, the steel plate enters UFC+ACC for swing cooling, and the final cooling temperature is 350℃. Then it is quickly taken off the line for stack cooling for more than 72 hours.

[0109] 5. Heat the steel plate to 865°C in a heat treatment furnace. Keep warm for 10 minutes, then cool it naturally to 550°C in the air. Then cool the steel plate to room temperature.

[0110] After destacking, the whole plate was tested for flaw detection, and the flaw detection performance was good. Tensile, impact performance, cold bending, Z-direction performance and strain aging performance tests were carried out on the 120mm surface, 1 / 4 position and 1 / 2 position. The metallographic inspection results of the steel plate 1 / 4 position are as follows: Figure 2 The morphology and distribution of the precipitates are shown in Figure 3 The specific performance is shown in Table 1.

[0111] Comparative Example 1

[0112] Normalized steel plate for wind power tower door frame with a thickness of 120mm. The designed mass percentage of the steel composition is as follows: C: 0.16%, Si: 0.28%, Mn: 1.69%, Nb: 0.050%, Al: 0.035%, Ti: 0.015%, N: 0.0038%, and the balance is Fe and unavoidable impurities.

[0113] The specific steps and parameters of the above steel plate production method are as follows:

[0114] 1. The continuous casting is 400mm thick continuous casting billet, and the continuous casting billet is pile-cooled for 72 hours.

[0115] 2. Before the continuous casting billet enters the heating furnace, thick-gauge hot-rolled ordinary plates should be arranged before and after it to ensure the self-adaptation of the rolling mill and the stacking effect and self-tempering of the steel plate.

[0116] 3. The continuous casting billet is heated in a stepping beam heating furnace, the furnace time is 349 minutes, and the billet temperature out of the furnace is 1192℃. Then a two-stage rolling process is adopted. The rough rolling adopts a low-speed large reduction process. The rolling force and rolling torque are set to the maximum allowable capacity of the rolling mill. After six passes, the intermediate waiting thickness is 180mm, and the surface is cooled with dephosphorized water in each pass during the rolling process; the intermediate billet is air-cooled on the roller table, and the non-recrystallization zone rolling begins after cooling to a surface temperature of 800℃. The rolling still adopts a large reduction strategy, rolling to a target thickness of 120mm, and the final rolling surface temperature is 792℃.

[0117] 4. After rolling, the steel plate enters the ACC for cooling, and the final cooling temperature is 650℃. Then it is quickly taken off the line for stack cooling for more than 72 hours.

[0118] 5. Heat the steel plate to 900℃ in a heat treatment furnace. Keep it warm for 10 minutes, then take it out of the heating furnace and let it cool naturally to room temperature in the air.

[0119] After destacking, the whole plate was tested for flaw detection, and the flaw detection performance was good. Tensile, impact performance, cold bending, Z-direction performance and strain aging performance tests were performed on the 120mm surface, 1 / 4 position and 1 / 2 position. The specific performance is shown in Table 1. Compared with the technology of the present invention, the strength of the steel plate is low and does not meet the strength requirement of 420MPa.

[0120] Comparative Example 2

[0121] Normalized steel plate for wind power tower door frame with a thickness of 120mm. The designed mass percentage of the steel composition is as follows: C: 0.16%, Si: 0.28%, Mn: 1.69%, V: 0.035%, Nb: 0.020%, Al: 0.007%, Ti: 0.015%, N: 0.012%, and the balance is Fe and unavoidable impurities.

[0122] The specific steps and parameters of the above steel plate production method are as follows:

[0123] 1. The continuous casting is 400mm continuous casting billet, and the continuous casting billet is pile-cooled for 72 hours.

[0124] 2. Before the continuous casting billet enters the heating furnace, thick-gauge hot-rolled ordinary plates should be arranged before and after it to ensure the self-adaptation of the rolling mill and the stacking effect and self-tempering of the steel plate.

[0125] 3. The continuous casting billet is heated in a stepping beam heating furnace, the furnace time is 349 minutes, and the billet temperature out of the furnace is 1200℃. Then a two-stage rolling process is adopted. The rough rolling adopts a low-speed large reduction process. The rolling force and rolling torque are set to the maximum allowable capacity of the rolling mill. After six passes, the intermediate waiting thickness is 180mm, and the surface is cooled with dephosphorized water in each pass during the rolling process; the intermediate billet is air-cooled on the roller table, and the non-recrystallization zone rolling begins after cooling to a surface temperature of 820℃. The rolling still adopts a large reduction strategy, rolling to a target thickness of 120mm, and the final rolling surface temperature is 812℃.

[0126] 4. After rolling, the steel plate enters the ACC for cooling, and the final cooling temperature is 650℃. Then it is quickly taken off the line for stack cooling for more than 72 hours.

[0127] 5. Heat the steel plate to 900℃ in a heat treatment furnace. Keep it warm for 10 minutes, then take it out of the heating furnace and let it cool naturally to room temperature in the air.

[0128] After destacking, the whole plate was tested for flaw detection, and the flaw detection performance was good. The tensile, impact performance, cold bending, Z-direction performance and strain aging performance tests were performed on the 120mm surface, 1 / 4 position and 1 / 2 position. The specific performance is shown in Table 1. Compared with the technology of the present invention, the strain aging performance of the steel plate is poor.

[0129] It can be seen from Table 1 that the yield strength of the steel plate is above 430MPa, the tensile strength is 530MPa and above, the single value of the impact energy at -40℃ is above 130J, and it has good strain aging impact energy, and the single value of the strain aging impact energy at -40℃ is above 100J.

[0130] Attached Figure 2-3 Detailed description:

[0131] Figure 2 is the metallographic structure diagram of the steel plate provided in Example 2, such as Figure 2 As shown, the microstructure of the steel plate is ferrite + pearlite, and the ferrite grain size is 7.5 μm to 11.5 μm.

[0132] Figure 3 is the steel plate precipitation phase diagram provided in Example 2, such as Figure 3 As shown, there are Nb-V composite precipitations in the precipitate phase, and a large number of V (C, N) precipitates have a size of 10nm to 20nm.

[0133] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0134] The embodiment of the present invention successfully avoids the addition of precious metal elements and reduces production costs by utilizing relatively cheap V elements and combining specific rolling and heat treatment processes.

[0135] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. It will be apparent to those skilled in the art that various modifications to these embodiments are possible, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown in the present application, but will conform to the widest range consistent with the principles and novel features applied for by the present application.

Claims

1. A normalized steel plate for a wind power tower door frame, wherein the chemical composition of the normalized steel plate comprises, by mass fraction: C： 0.15%~0.18%, Si: 0.20%~0.50%, Mn: 1.60%~1.80%, Nb: 0.02%~0.03%, V: 0.025%~0.045%, N: 0.0050%~0.0080%, P≤0.010%, S≤0.003%, carbon equivalent: 0.44%~0.46%, Alt≤0.010%, and Fe; Among them, 6≤(V+Nb) / N≤10, where (V+Nb) / N represents the ratio of the sum of the mass fractions of V and Nb to the mass fraction of N.

2. The method according to claim 1, characterized in that The microstructure of the normalized steel plate is ferrite and pearlite.

3. The method according to claim 2, characterized in that The grain size of the ferrite is 7.5 μm to 11.5 μm.

4. The method according to claim 1, characterized in that: The normalized steel plate meets at least one of the following properties: Yield strength>430MPa; Tensile strength>530MPa; -40℃ impact energy single value>130J; -40℃ strain aging impact energy single value>100J.

5. A method for preparing a normalized steel plate according to any one of claims 1 to 4, the method comprising: Obtaining molten steel having the chemical composition; Continuously casting the molten steel to obtain a continuously cast billet; The continuous casting billet is sequentially subjected to first batch cooling, heating, rough rolling and finish rolling to obtain a semi-finished steel plate; performing a first cooling on the semi-finished steel plate; The semi-finished steel plate after the first cooling is sequentially subjected to second stack cooling, normalizing heat treatment and second cooling to obtain a finished steel plate.

6. The method according to claim 5, characterized in that The thickness of the continuous casting billet is 300 mm to 400 mm.

7. The method according to claim 5, characterized in that The temperature of the first cooling is 400° C. to 700° C., and the time of the first cooling is >36 hours; and / or, The heating temperature is 1140°C to 1170°C, and the heating time is ≥ 240 min; and / or, The rough rolling is low-speed high-reduction rolling, the rolling speed of the rough rolling is 0.5m / s to 1.0m / s, the single-pass reduction of the rough rolling is ≥35mm, and the intermediate warm-up thickness of the rough rolling is 2 to 3 times the thickness of the finished steel plate; and / or, The start rolling temperature of the finishing rolling is 870°C to 850°C, and the final rolling temperature of the finishing rolling is 840°C to 850°C.

8. The method according to any one of claims 5 to 7, characterized in that: The continuous casting billet is sequentially subjected to first batch cooling, heating, rough rolling and finish rolling to obtain a semi-finished steel plate; The continuously cast billet is subjected to first pile cooling, heating, dephosphorization, rough rolling and finish rolling in sequence to obtain a semi-finished steel plate; wherein the rough rolling is a multi-pass rough rolling, and in the interval between two adjacent passes of the rough rolling, the dephosphorization water is used to cool the continuously cast billet.

9. The method according to claim 5, characterized in that The first cooling is swing cooling, the start cooling temperature of the first cooling is 790°C to 820°C, the final cooling temperature of the first cooling is 350°C to 400°C, and the cooling rate of the first cooling is 15°C / s to 25°C / s.

10. The method according to claim 5, characterized in that The second stack cooling temperature is greater than 300° C., and the second stack cooling stacking time is greater than 36 hours; and / or, The heating temperature of the normalizing heat treatment is (Ac3+5)°C to (Ac3+10)°C, the temperature rise coefficient of the normalizing heat treatment is 2.3min / mm to 2.5min / mm, and the holding time of the normalizing heat treatment is 5min to 10min; and / or, The second cooling is to naturally cool the semi-finished steel plate to 550° C. to 580° C., and then to stack cool the semi-finished steel plate to room temperature.

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