Study Of The Second Phase Precipitation Behaviors And Welding Microstructures Transformation In Titanium Microalloyed Q355C Steel

Ti microalloyed steel is a high strength low alloy structural steel with good strength、good plasticity、toughness and processability.A domestic steel plant adopts the idea of high Ti medium Mn to produce Ti microalloyed Q355 steel,hoping to make up for the lack of strength caused by the reduction of Mn content through the precipitation of Ti,but many practical problems such as impact property and elongation instability appear in the steel plate,so the subject for the production of high Ti medium Mn Q355C steel,mainly to explore the precipitate behaviors of the second phase particles of Q355C steel,and the welding microstructures transformation of Q355C steel through thermal simulation test.The following conclusions were drawn:（1）The results of thermodynamic software JMat Pro theoretical calculation and SEM and EDS detection of refining,continuous casting and rolling stages show that the second phase containing titanium in titanium microalloyed Q355C steel mainly exists in Ti N and Ti C forms,and there are no titanium oxide and Ti₄C₂S₂.（2）The further study of the second phase composition,lattice,type and size of the two rolled samples with different thicknesses again shows that the second phase particles in titanium microalloyed Q355C steel are mainly Ti N and Ti C,and there is no Ti₄C₂S₂.The particles of 35 mm thick 6#containing titanium are Ti N with fcc structure,and of 69particles with average size of 113 nm;25 mm thick 7#including 64 large squares with average size of 123 nm and 68 small circular particles with average size of 7.2 nm,both of which are Ti C with fcc structure.（3）Two rolling samples of different thicknesses were tensile and impact tested.The mechanical properties results show that the 25 mm sample has better comprehensive mechanical properties compared with the 35 mm sample,and the relatively small microstructures is one of the main reasons for its excellent comprehensive properties,and it is appropriate to choose the 25 mm thick rolled sample for subsequent thermal simulation experiments.（4）By using the thermal simulator to study the microstructures transformation at different cooling rates,and the mechanism of the intragranular acicular ferrite（IAF）nucleation is deeply studied.The results show that the IAF can be obtained in a large range of cooling rate,only the inclusion particles distributed in the crystal can induce the IAF nucleation,and only the appropriate size of the austenite grain（about 190μm）is beneficial to promote the IAF nucleation.The inclusion plays an important role in IAF nucleation,so in order to further study the influence of different titanium content on the precipitation of inclusions in the HAZ,the thermodynamic software Fact Sage is used for calculation.The results indicate the precipitation temperature Ti N>Ti C.With the increase of titanium content,the precipitation amount of Ti N is almost not affected,but the precipitation temperature increases with the increase of Ti content,and the precipitation temperature and precipitation amount of Ti C also increase with the increase of Ti content.Therefore,for the Q355C（Ti 0.06%）,the limiting factor for Ti N formation is N content,and Ti has reached saturation with the Ti content,0.06%.