Application of new technology in steel structure welding
3 Low alloy high strength steel welding technology
In 2004, the low-alloy high-strength steel ASTMA913Gr60 (equivalent to Q420) was successfully used in Beijing Xinbaoli Building. After two years of development, at present, several steel structures in China have used high-strength steel, such as the National Stadium (Bird's Nest) using domestic Q460E. -Z35 steel, the maximum thickness is 110mm, the national swimming center (water cube) project uses domestic Q420C steel, CCTV new site project uses Q390D, Q420D-Z25, Q460E-Z35 grade steel, high-strength steel in the construction steel structure Application, led to the development of high-strength steel welding technology.
(1) Ways to achieve good performance in high-strength steel for construction steel structures:
1 alloy strengthening: through the fine-grain strengthening and precipitation strengthening of microalloying elements, the strength and toughness of the steel sheet are improved; the grain and uniform structure are refined by normalizing, and the plasticity and toughness of the steel sheet are further improved. Domestic high-strength steel (medium plate) is mainly manufactured by this method;
2 tissue strengthening (such as quenching + tempering): the heating temperature after rolling exceeds the phase transformation recrystallization temperature of 30 ~ 50 ° C, the quenched supersaturated solid solution formed by water cooling is unstable structure, high strength and hardness. Subsequent high temperature tempering at 600 °C can decompose and soften the quenched solid solution to meet the requirements of plasticity and toughness, also known as quenching and tempering treatment.
3 Controlled Rolling and Controlled Cooling Process (TMCP): Strictly control the process of steel plate cooling and thickness reduction, and finish rolling at a temperature close to or below the temperature at which ferrite begins to form (Ar3910 degrees Celsius). Its microstructure and mechanical properties cannot be obtained by heat treatment. This rolling method can obtain higher strength and good weldability at a lower carbon equivalent;
4 quenching + self-tempering controlled rolling (QST): Rolled H-beam of Luxembourg steel mill (Gr60 steel used in the project of Xinbaoli Building), self-tempering after quenching using heat dissipation in the middle of section, is special for TMCP application. For example, Gr65 steel has a quenching temperature of 871 degrees Celsius and a self-tempering temperature of 593 degrees Celsius. Its strength is high and its weldability is good.
(2) Principles for matching welding consumables for high strength steel
1 strong matching, strong node weak rod: welding material, the strength, plasticity, impact toughness of the deposited metal is higher than the minimum value specified in the parent metal standard. The comprehensive performance requirements of the welded joints (welds and heat-affected zones) meet the minimum values ​​specified in the parent metal standard;
2 Considering weld plasticity: When welding thick plates, the welding consumables are selected according to the strength after thickness effect. When the degree of restraint is large, low-strength welding consumables can be used below 1/4 plate thickness;
3 to meet the impact toughness requirements: must focus on the toughness of the welding consumables to make the weld and heat affected zone toughness meet the standard requirements of steel.
(3) High-strength steel weldability evaluation method
1 carbon equivalent calculation method;
2 The highest hardness test method for heat affected zone;
3 bolt test critical fracture stress assessment method.
(4) Minimum preheating temperature determination method
1 Crack test control: Determine the minimum preheating temperature according to the crack test of the oblique Y groove sample;
2 hardness control: according to a certain carbon equivalent of steel, the thickness of the T-joint fillet weld heat affected zone hardness reaches 350 HV corresponding cooling rate (540 degrees Celsius) to determine the welding line energy;
3 According to the crack sensitivity index, the thickness range of the plate, the degree of restraint, and the diffused hydrogen content of the deposited metal, the minimum preheating temperature is determined. 4 The minimum preheating temperature is determined according to the heat input of the joint, the cooling time T8/5 and the specific curve of the steel.
(5) Welding quality control
1 Control heat input and cooling speed: control welding current, voltage, welding speed and cooling time (t8/5) (preheating, afterheating) of deposited metal in the range of 800 degrees Celsius to 500 degrees Celsius;
2 Control the carbon/sulfur/phosphorus/nitrogen/hydrogen/oxygen content in the weld: select high-quality alkaline low-hydrogen welding consumables, and fully protect the molten pool metal (short arc, limited swing, stable inclination) with good operation method;
3 Stress and deformation control: welding method with high energy density and low heat input, such as gas shielded welding; small line energy, multi-layer multi-pass welding; reducing the angle and gap of welding groove, reducing the filling amount of deposited metal; Symmetrical, unidirectional welding is used for symmetrical beveling; long welds should be reflowed in stages or multi-person welding at the same time; use of jump welding to avoid deformation and stress concentration.
In short, for the welding of high-strength steel, according to the strengthening mechanism and supply state of the steel itself, comprehensive consideration of its performance requirements, reasonable selection of welding materials and test methods to evaluate its weldability, and the development of a reasonable welding process to guide the actual welding Production, such as the national stadium engineering application of Q460E-Z35 steel is microalloyed, its carbon equivalent Ceq is as high as 0.47%, therefore, the welding of this steel should mainly consider measures to reduce its cold cracking tendency, and in the new The Baoli Building project uses high-strength steel strengthened by QST technology. The carbon equivalent Ceq of this steel is only 0.36%. The weldability is quite good, but since it uses the quenching and self-tempering process, it should be welded. Strictly control the interlayer temperature and welding line energy to prevent the joint from weakening.
4 thick plate welding technology
The steel plate of construction steel structure has been used more and more in large quantities. For example, the thickness of rolled H-shaped steel wing used in Beijing Xinbaoli Building project is 125 mm (ASTMA 913Gr60), and the maximum thickness of engineering steel for National Stadium (Bird's Nest) is 110 mm. (Q460E-Z35), a large number of steel structure projects use thick steel plate, which promotes the development of thick steel plate welding technology, and also enriches the range of construction steel. At present, the current domestic standards such as GB/T1591-94 "low alloy high strength structural steel" The thickness of the steel plate specified in YB4104-2000 "Steel Sheet for High-rise Building Structure" is only 100 mm, so the use of ultra-thick steel plate also laid the foundation for the revision of the corresponding standard.
The key to welding thick steel plates is to prevent cracks and reduce deformation due to welding. The following measures should be taken:
1 Select a reasonable groove form, such as double U or X groove as much as possible. If only one side welding is used, a small angle and a narrow gap should be used to reduce the welding shrinkage under the premise of ensuring the penetration. Improve work efficiency and reduce welding residual stress;
2 select a reasonable preheating and interlayer temperature;
3 symmetric welding, scientific arrangement of welding sequence;
4 Do a good job of post-heating and heat preservation.
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