Research On Process, Microstructure And Mechanical Properties Of Nb-bearing Bridges H-beam Steel

The demand is increasing on the property of bridge steel in the market with the modernization of bridge design and construction. In order to adapt to the adverse environment, the mechanical properties of H-beam using for bridge can be improved with the help of microalloying technology and thermo-mechanical controlling process.In the thesis, the420MPa high strength low alloy H-beam using for bridge was used as the research object, thermal-mechanical simulating technology and hot rolling process were applied, continuous cooling transformation and thermo-mechanical controlling process of test steel were researched.The main contents and results are as follows:(1) The single-pass compression was tested for the experimental steels. The influence of the deforming temperature, strain value and strain rate on the austenite recrystallization were investigated, and the models were established, such as the mathematical models of deformation resistance, the mathematical and critical strain model of dynamic recrystallization. The double-pass compression was also tested for the experimental steels. The static softening curves were investigated, and the effect of different process parameters on static recrystallization behavior of austenite were researched, which provided the theoretical and practical basis for controlled rolling.(2) The dynamic and static CCT curves of test steel at various cooling rates were determined using dilatometric method in the thermal simulation machine of MMS-300. And the continuous cooling transformation of test steel was investigated with the help of metallographic observation.(3) The hot rolling test was carried out to study the effect of different deformation amount and finishing rolling temperature on the microstructure and mechanical properties, the results showed that the ferrite grain can be refined by increasing deformation and lowering finishing rolling temperature. When the finishing rolling temperature was between800℃and930℃, the strength was decreased and the elongation was increased with the finishing rolling temperature increasing. And the low temperature impact property were both qualified. (4) The hot rolling and cooling test showed that compared with laminar and air cooling, the strength of Q420qE after ultra-fast cooling(UFC) was higher. The finer bainitic-ferrite plate was increased, the orientation became straighter, the dislocation density was higher, the strength was increased and the elongation was decreased with the finishing cooling temperature after UFC decreasing.