Study On The Weldability Of High Strength Bridge Steel

High strength bridge steel has been studied widely and substantive patents have been obtained in foreign countries at present. However, the study on high strength bridge steel in China has just been started recently. So, the development of high strength bridge is urgent to carry out in China. With the support of the project of "development of steels" taken on by Capital Steel Company and Northeastern University, the weldability of a high strength bridge steel Q460q was systematically studied in this thesis, and the main work is as follows:(1) In order to determine reasonable welding parameters, and to estimate the service weldability and fabrication weldability, Q460q was welded under different parameters using submerged-arc welding during which heat input is high. The results proved that, Q460q has high service weldability. The strength of the weld joint is not lower than the base metal, good toughness is obtained at -60℃～20℃, and the DATT of the welding line, bond and heat affected zone are low. In addition, cold crack is not formed during welding, proving the good fabrication weldability of Q460q.(2) The microstructure of heat affected zone(HAZ) in Q460q and Q460qNH with different heat input was studied through thermo mechanical simulation experiments. The results showe that bainite including lathing and granular formed in the coarse grain heat affected zone(CGHAZ) when t8/5 is in the range of 30s～200s. The morphology, distribution and quantity of the M-A constituents in bainite changed with t8/5 and has obvious effect on the toughness of coarse grain zone. The original austenite grain grows with the increase of t8/5-The increase of effective grain size deteriorate the toughness. Thus, in order to improve the toughness, the grain size of CGHAZ should be controlled with appropriate method.(3) The SH-CCT curve was determined, and it is greatly helpful to study the physical metallurgy during welding and to set down appropriate welding parameters. (4) The fatigue S-N curve of the welding joint, the material constant C, exponent m and the crack threshold value AKth in Paris Formula were determined with sine wave (R>0), and the initiation mechanism of fatigue crack was also investigated. The results showe that the fatigue value was 470MPa. The fatigue crack under higher stress amplitude originated from some defect at the surface of the sample, and originated from inclusion under lower stress amplitude. The AKth of welding line in the welding joint is higher than that of the base metal, while crack extension rate is lower than that of HAZ and base metal. The microstructure changes as follows with the increase of cycle index:dislocation tangleâ†'dislocation wall formedâ†'dislocation cell formedâ†'subgrain formed. Then the crack developes when the subgrain boundary split.(5) The CGHAZ embrittlemented as t8/5>80. To improve the toughness of the CGHAZ, the effect of oxide on the physical metallurgy in low alloy high strength steel was firstly studied. Dispersed second-phase CaO and CaS formed by low Ca treatment. On one hand, The dispersed and steady CaO and CaS would pin the austenite boundary migrating at 1350～1450℃. As a result, the microstructure is refined in coarse grain zone and the toughness is improved. On the other hand, acicular ferrite can nucleate at larger CaO, leading to the formation of acicular ferrite with high toughness. It is observed that when t8/5 is 80s and 200s(E=87 and 117kJ/cm), the original austenite grains are refined evidently. The growing rate of grains slow down with the increase of t8/5, and the toughness is improved obviously.(6) The microstructure and properties as well as the growth tendency of austenite grains in HAZ of micro-added Ca steel were investigated by real welding and thermo mechanical simulation. Experiments were carried out under different conditions. The results showe that as t8/5= 8s,20s,40s,80s, and 200s, the average austenite grain diameter in CGHAZ is about 20μm,40μm,60μm,80μm and 110μm, respectively. Compared with the low alloy high strength steel, the austenite grain growth rate in coarse grain zone decrease dramatically in micro-added Ca steel. Grain refinement in HAZ is apparently effective in micro-added Ca steel.