Investigations Of TMCP Procedure And Weldability For Q420 Weathering Steel

The effects of different TMCP parameters and welding thermal cycle conditions on the microstructure and mechanical properties of a Q420 weathering steel were investigated,and the experiments were conducted in the Gleeble-3500 thermal simulation testing machine in this thesis.Based upon the continuous cooling transformation of the hot deformed austenite,the dynamic CCT curve was studied.Above test results provide guidances for the controlled cooling process of the Q420 weathering steel.On the basis of this analysis,the crucial stages for TMCP of the Q420 weathering steel were simulated,covering the cooling rate at 1-15℃/s,the fianl rolling at 820-880℃,and the self-tempering temperature at 530-680℃.The effect of the different TMCP parameters on the microstructure and mechanical properties was described.To study the microstructural evolution and mechanical properties in CGHAZ of the testing steel,the welding continuous cooling transform(SHCCT)map was established,and the simulated thermal cycle experiments with different heat inputs was carried out.The result indicates that:The continuous cooling transformation of the hot deformed austenite indicates that: with the cooling rate in the range of 0.5-1℃/s,the microstructure was polygonal ferrite and pearlite,and the hardness was low.When the cooling rate increased to 2.5℃/s,the granular bainite ferrite appeared and the hardness increased quickly.Along with the further increasing of cooling rate,the amount of granular bainite ferrite increased gradually and ferrite grains refined significantly.With the cooling rate in the range of 20-30℃/s,the lath bainite ferrite started to appear.The microstructure and mechanical properties change rule under the condition of different rolling and cooling process indicates that: if the tensile properties including the yield strength,tensile strength and YR regulated by the ranges of ≥420MPa,≥540MPa and ≤0.85,respectively should be controlled simultaneously,the parameters of the cooling rate at 10-15℃/s,the fianl rolling at 860-880℃,and the self-tempering temperature at 580-630℃ were optimized for the TMCP procedure.And the type of microstructure was fine granular bainite ferrite and acicular ferrite.Along with the increasing of cooling rate,or the ruducing of fianl rolling and self-tempering temperature,the grain size trended refining.Study on the welding thermal cycle indicates that: along with the heat input increasing,the type of microstructure changed remarkably,the type of microstructure evolved from lath bainite ferrite to granular bainite ferrite,and the grain coarsed obviously.Under the very fast cooling rate conditon,the highest hardness of CGHAZ was 303(HV0.3),showing low tendency of hardenability.With increasing the heat input,the low temperature impact value decreased significantly.When the heat input in the range of 10-30kJ/cm,the impact property of CGHAZ meeted the requirement of Q420 weathering steel.