Effects of peak temperature on microstructure and mechanical properties of coarse grain zone of 780HE complex phase steel
[Objective]The purpose of this paper was to systematically study effects of peak temperature on microstructure,precipitates,and mechanical properties of coarse grain zone of 780HE complex phase steel.[Methods]A thermomechanical simulator Gleeble-3800 was used to simulate thermal cycles experienced by 800 MPa-grade complex phase steel 780HE under different welding heat inputs during welding process.[Results]The coarse grain zones were composed of bainite under different peak temperatures.When peak temperature rose from 1140℃ to 1320℃,initial transformation temperature of austenite decreased,making content of lath bainite increase from 10％to 95％,and dislocation density increase from 0.96×106 mm-2 to 1.12×106 mm-2.Precipitates of coarse grain zones were mainly composed of nearly circular composite precipitates(Ti,Nb)C,accompanied by several square precipitates(Ti,Nb)(C,N).When peak temperature was 1210℃ or above,(Ti,Nb)C with size of 2～10 nm disappeared,and density of(Ti,Nb)C with large size decreased to below 7 μm-2.As peak temperature increased from 1140℃ to 1210℃ and 1320℃,tensile strength were 750 MPa,744 MPa and 789 MPa,yield strength were 661 MPa,652 MPa and 705 MPa,and percentage elongation after fracture was 12.2％,12.4％and 10.4％,respectively.Impact energy absorption were 64.17 J,18.55 J and 5.64 J,respectively,and impact fracture surface transformed from ductile fracture to brittle fracture.[Conclusion]Peak temperature had no significant effect on microstructure and types of precipitates in coarse grain zone.As peak temperature increased,content of lath bainite increased,dislocation density increased,and size of precipitates increased.Yield strength first slightly decreased and then significantly increased,opposite to the trend of percentage elongation after fracture,while impact toughness significantly decreased.
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