Steel for wind power

Among the steel used in wind power projects, steel plates are used for wind power towers. The special requirements for steel quality of towers are mainly related to the regional environment. In the "Three Norths" region, which accounts for 76% of the country's wind power installed capacity, the lowest temperature in winter is below minus 30 degrees. When selecting low-alloy structural steel materials for low-temperature towers, technical measures for low-temperature brittle fracture are required for welds. The performance requirements of steel materials are to prevent low-temperature brittle fracture, and appropriate treatment methods are required to enhance the material's resistance to multiple impacts, avoid stress concentration, and avoid large impact loads under low temperature conditions.

Technical requirements: The main materials of the tower are: thick plates of Q345E, Q345D, and Q345C, and the flange material is Q345D or Q345E, which is a ring forging. The main manufacturers of Q345 steel for wind power towers include Laigang, Wugang, Jiugang, Xingcheng Special Steel, Angang and Baosteel. In 2011, Wugang produced 501,000 tons of steel for wind power and Jiugang produced 250,000 tons. The production technology is not difficult and all large-scale thick plate manufacturers in China have the production capacity.

Technology and process: Taking a certain enterprise producing Q345E steel for wind power The first is to analyze from the chemical composition Q345E is a low-alloy high-strength structural steel with a low grade and low technical requirements. However, how to use the most economical means to produce products that meet standards and user requirements is one of the fundamental starting points of process design.

Nb, Ti, and V are the most commonly used microalloying elements. The effects of the above three elements on grain boundaries decrease in turn. In low-alloy high-strength steel, the effect of composite microalloying is greater than the sum of adding a certain element alone. Nb, Ti, and V can all be precipitated in austenite or ferrite. Because the solubility in austenite is large and the diffusion rate is small, the precipitation in austenite is slower than in ferrite, and deformation can accelerate the precipitation process. Generally, the precipitation in austenite should be minimized, and more alloying elements should be kept in the solid solution and left to precipitate in ferrite. This can be achieved by alloying to increase the solubility of the micro-alloying elements in austenite. For example, Mn or Mo is added to Nb-containing steel. Which element to use to strengthen Q345E is the primary consideration.

Since the price of Ti is the lowest among the three alloying elements of Nb, V, and Ti, the cost of producing Q345E by using Ti microalloying is low, which can achieve the effect of fine grain strengthening and the purpose of reducing costs. However, the Ti yield is the most unstable, and the smelting operation technology is more difficult.

Taking all the above factors into consideration, the three microalloying processes of Al, Ti, and V were used for Q345E production trials. Rolling and cooling control

Products with a thickness of ≤9mm are rolled conventionally, and products of other specifications are rolled in two stages. The temperature-waiting thickness of the controlled rolling is hi=(1.5~3.0)×h0, where h0 is the finished product thickness. The first rolling temperature is 1050~1100℃, and the second rolling temperature is 780~820℃. The cooling rate is controlled between 5~15℃/s according to different thicknesses, and the final cooling temperature is controlled between 670~700℃.

Development of structural steel plates for wind power towers: Wind power towers, also known as tower barrels, are used to support wind power generators. They mainly bear loads including peak loads, unit weight loads, and eccentric moments caused by unit gravity center offset. Due to the influence of regional, climate, load, and environmental conditions, the material requirements for wind power towers mainly include: chemical composition, mechanical properties, weather resistance, corrosion resistance, welding performance, etc. The technical standards for the supply technical conditions of wind tower structural parts designed according to European standards are mainly: EN10025, EN10163, EN10204, EN10029, GD009079, etc. The steel grades include:

S235JR, S235J0, S355J2+N, S355J2, S355J0, S355JR, S275JR, etc. The steel plate must pass the CE quality certification, and there are specific requirements for the carbon equivalent of the steel grade, flaw detection, etc. The requirements for steel plate materials in the design of domestic wind power towers mainly refer to the European standard series, and the corresponding national standard grades are used, such as: Q345D, Q345E, etc., and the corresponding technical conditions need to be added in the delivery requirements.With the continuous improvement of wind power equipment technology and the increase in the capacity of single-unit equipment, the requirements for wind power tower materials will become increasingly stringent. Considering that the wind power tower structure is subjected to heavy loads, low temperature resistance, strong earthquakes and various harsh environments (fire, wind, severe corrosion, etc.),

higher requirements are placed on the performance of steel. The development trend is:

1) The plate thickness continues to increase, the plate strength continues to increase, and the welding performance requirements are more perfect;

2) Increase the variety of steel plates and expand the scope of use. Since the production and on-site installation are carried out in winter, welding at low temperatures (-20 degrees) is a challenge to the performance of steel;

3) The performance and dimensional accuracy need to be further improved. It is required to meet the design requirements in terms of surface quality, internal quality, size and accuracy;

4) Improve weather resistance and corrosion resistance.

The metallurgical quality control requirements for steel used in wind power towers are relatively high. At present, foreign countries generally use furnace refining, continuous casting process with electromagnetic stirring and controlled rolling and controlled cooling technology. Its characteristics are that the main components such as C, Si, Mn and microalloys in the steel can be controlled within a narrower range than usual, the uniformity of the steel is good and the fluctuation range of product performance is small; the content of impurities such as P, S, O, and N is reduced and controlled, the number of non-metallic inclusions in the steel is reduced, and the inclusions are denatured to improve the comprehensive performance of the steel and improve the performance of the steel. Therefore, the metallurgical process control of this type of steel is very critical.

Standardization requirements for structural steel plates for wind power towers: At present, the design of wind tower steel in wind power equipment mainly adopts European standard structural steel plates, and its technical conditions have their own characteristics in terms of chemical composition, mechanical properties, quality, tolerance, inspection and performance. Product users pay more and more attention to the applicability, personalization and cost-effectiveness of the products during use. Although major domestic steel plate manufacturers can produce products that meet user requirements, there is currently no corresponding product standard in China, which affects the promotion and application of domestic structural steel plates for wind power towers. In response to the above problems, the Steel Plate and Strip Branch of the National Steel Standardization Committee organized major domestic wind power tower structural steel plate manufacturers to conduct standardization demonstration of various technical indicators of the products, and successfully established the project in the country.

Standardization of the main technical content of structural steel plates for wind power towers

Name method of the brand: The brand consists of several parts such as the Chinese pinyin letters representing the yield strength, the yield strength value, the first Chinese pinyin letter of "wind tower", and the quality grade symbol. For example: Q345FTD.

4.2 Steel grade

Including seven strength grades with yield strength of 235, 275, 345, 420, 460,

550, 690, each strength level includes partial grades of five quality grades of B, C, D, E, and F.

4.3 Smelting method

It is stipulated that the steel plate is smelted by a converter or an electric furnace, and should be refined outside the furnace. The purity of the molten steel is guaranteed.

4.4 Delivery Status

Different delivery status are specified for each strength grade, including hot rolling, controlled rolling, normalizing/normalizing rolling, TMCP, TMCP+

tempering, quenching + tempering, etc., which are in line with the current product development and production practices.