Application of the latest technology in steel stru

Application of new technology in steel structure welding (III)

3 low alloy high strength steel welding technology

in 2004, low alloy high strength steel astma913gr60 (equivalent to Q420) was successfully used in Beijing New Poly building project. After two years of development, at present, several steel structure projects in China have used high strength steel, such as the National Stadium (Bird's Nest) uses domestic q460e-z35 steel, with a maximum plate thickness of 110mm, The National Swimming Center (Water Cube) project uses domestic Q420C steel, and the CCTV new site project uses q390d, q420d-z25, q460e-z35 grade steel. The wide application of high-strength steel in building steel structures has driven the development of high-strength steel welding technology

(1) ways to obtain good properties of high-strength steel for building steel structures:

① alloy strengthening: improve the strength and toughness of steel plates through fine grain strengthening and precipitation strengthening of microalloyed elements; The plasticity and toughness of the steel plate are further improved by normalizing to refine the grain and uniform structure. Domestic high-strength steel (medium and thick plate) is mainly manufactured by this method

② structure strengthening (such as quenching + tempering): the heating temperature after rolling exceeds the phase transformation recrystallization temperature by 30 ~ 50 ℃, and the quenching supersaturation generated after water cooling will not pollute the groundwater. The solid solution is an unstable structure with high strength and hardness. Then high temperature tempering at 600 ℃ can decompose and soften the quenched solid solution to meet the requirements of plasticity and toughness, which is also called quenching and tempering treatment

③ controlled rolling and cooling process (TMCP): strictly control the process of steel plate cooling and thickness reduction, and complete the final rolling at a temperature close to or lower than the temperature at which ferrite begins to form (ar3910 ℃). Its microstructure and mechanical properties cannot be obtained by heat treatment. This rolling method can obtain high strength and good weldability under low carbon equivalent

④ quenching + self tempering controlled rolling (QST): Rolled H-section steel of Luxemburg steel works (GR60 steel was used in the new poly building project). After quenching, the temperature in the middle of the section is used for self tempering, which is a special application of TMCP. For example, the quenching temperature of gr65 steel is 871 ℃, and the self tempering temperature after 5 seconds is 593 ℃. It has high strength and good weldability

(2) selection principle of welding materials for high-strength steel

① strong matching, strong node and weak rod: the strength, plasticity and impact toughness of the deposited metal of the welding material are higher than the minimum value specified in the base metal standard. All properties of welded joints (welds and heat affected zones) are required to meet the minimum value specified in the base metal standard

② give consideration to weld plasticity: when welding thick plates, select welding materials according to the strength after thickness effect, and when the node restraint is large, use low strength welding materials below 1/4 of the plate thickness

③ meet the requirements of impact toughness: the toughness of welding materials must be selected to make the toughness of weld and heat affected zone meet the standard requirements of steel

(3) evaluation method of weldability of high-strength steel

① carbon equivalent calculation evaluation method

② evaluation method of maximum hardness test in heat affected zone

③ critical fracture stress evaluation method of pin test

(4) determination method of minimum preheating temperature

① crack test control: determine the minimum preheating temperature according to the crack resistance test of inclined Y-groove samples

② hardness control: according to the steel with a certain carbon equivalent, the hardness of the heat affected zone of the T-joint fillet weld with different plate thickness reaches 350hv, and the corresponding cooling rate (at 540 ℃) is checked to determine the welding line energy

③ determine the minimum preheating temperature according to the crack sensitivity index, plate thickness range, restraint grade, diffusive hydrogen content of deposited metal ④ determine the minimum preheating temperature according to the joint heat input, cooling time t8/5 and the specific curve of steel

(5) welding quality control

① control heat input and cooling speed: control welding current, voltage Welding speed and cooling time (t8/5) of deposited metal in the range of 800 ℃ ~500 ℃ (preheating and post heating)

② control the content of carbon/sulfur/phosphorus/nitrogen/hydrogen/oxygen in the weld: select high-quality alkaline low hydrogen welding materials, and adopt good operation practices to fully protect the metal in the weld pool (short arc, limited swing, stable inclination)

③ stress and deformation control: select welding methods with high energy density and low heat input, such as gas shielded welding; With small line energy, multi-layer and multi pass welding; Reduce the angle and gap of welding groove, and reduce the filling amount of deposited metal; Adopt symmetrical groove and weld symmetrically and in turn; Long welds shall be back welded in sections or welded by multiple people at the same time; Skip welding is used 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, comprehensively consider its performance requirements, reasonably select welding materials and test methods, evaluate its weldability, and formulate reasonable welding processes to guide the actual welding production. For example, q460e-z35 steel used in the National Stadium project adopts microalloy strengthening, and its carbon equivalent CEQ is as high as 0.47%. Therefore, The welding of this steel should mainly consider taking measures to reduce its cold cracking tendency, while the high-strength steel strengthened by QST process is used in the new poly building project. The carbon equivalent CEQ of this steel is only 0.36%, and the weldability is quite good. However, because it uses the quenching and self tempering process, the interlayer temperature and welding line energy, which are the main ways of automobile energy conservation and emission reduction, should be strictly controlled during welding, Prevent the joint from weakening

4 thick plate welding technology

medium and thick steel plates of building steel structures are used more and more. For example, the thickness of rolled H-shaped steel wing plates used in Beijing New Poly building project reaches 125mm (astma913gr60), and the maximum plate thickness of steel used in National Stadium (Bird's Nest) project reaches 110mm (q460e-z35). A large number of steel structure projects use thick plates, which promotes the development of thick plate welding technology and enriches the range of construction steel, At present, current domestic standards such as gb/t "low alloy high strength structural steel" and Yb "steel plate for high-rise building structures" stipulate that the maximum thickness of steel plate is only 100 mm, so the use of ultra thick steel plate has also laid the foundation for the revision of the corresponding standards

change the experimental machine to be suitable for torque testing of various products

the key to thick steel plate welding is to prevent cracks and reduce deformation caused by welding, and the following measures should be taken:

① select a reasonable groove form, such as double u or X groove as far as possible. If only one side welding is available, small angle and narrow gap groove should be used on the premise of ensuring penetration, so as to reduce welding shrinkage, improve work efficiency Reduce welding residual stress

② choose reasonable preheating and interpass temperature

③ weld symmetrically and arrange the welding sequence scientifically

④ carry out post heating and insulation treatment