Study On Microstructure,Mechanical Properties And Spot Welding Properties Of High Property Microalloyed Cold Rolled Dual Phase Steel

Cold-rolled dual-phase steel is widely used in automotive steel because of its low yield strength,high initial work hardening rate,and good matching of strength and plasticity.In order to meet the needs of energy saving,consumption reduction and emission reduction,automotive steel is continuously developing in the direction of high strength.For cold-rolled dual-phase steel,as the strength increases,it is more difficult to control the comprehensive properties such as plasticity and formability,and at the same time,defects such as splashing and unmelting are likely to occur in the actual spot welding process.In this paper,two kinds of experimental steels containing Nb and Ti with bainite(1#)and ferrite-pearlite(2#)as initial microstructures were obtained by controlling the hot rolling process,and the ferrite recrystallization and austenite transformation in the annealing process of the two-phase region were studied after cold rolling.On this basis,the influence of initial microstructure and two-phase annealing process parameters on the mechanical properties of cold-rolled dual-phase steel was studied.The optimum spot welding parameters of dual-phase steel were explored and played the certain guiding significance.The main research contents and conclusions of this paper are as follows:(1)The effect of initial microstructure and heating rate on ferrite recrystallization has been investigated.The results show that the difference in initial organization will lead to different processes of recrystallization during annealing.The deformation storage energy of 1#experimental steel is larger,and the time required for recrystallization to complete is shortened.Compared with heating at 5℃/s,the recrystallization activation energy at 80℃/s is reduced,and the recrystallization process is accelerated.(2)The effect of heating rate and initial microstructure on austenite transformation kinetics was studied.The results show that the phase transition onset temperature Ts increases with the increase of the heating rate of 0.5℃/s,5℃/s,80℃/s,the end temperature Tf decreases,and increasing the heating rate will shorten the phase transition time and reduce the phase change temperature range.When heated at 80℃/s,martensite is prone to band-like distribution.Through recrystallization treatment,austenite can be nucleated along the polygonal ferrite grain boundary during annealing,so that martensite of uniform distribution can be obtained for 1#steel.Similarly,the recrystallization treatment promoted the nucleation of austenite in the 2#experimental steel at the recrystallized ferrite-ferrite grain boundary,which improved the uniformity of martensite distribution.(3)The effect of continuous annealing parameters on mechancial properties was investigated.The results show that the tensile strength of 1#steel is increased by 56MPa when heated to 760℃ at 80℃/s compared with the heating rate of 5℃/s.And the tensile strength of 2#steel increased by 40MPa when heated to 790℃ at 80℃/s,reaching 1087MPa and 1026MPa,respectively.(4)The optimal spot welding parameters of 1#and 2#steel are as follows.The welding current is 7.5kA and 8.0kA,welding time is 20cyc and 24cyc and electrode pressure is 4.0kN and 4.0kN,respectively.The maximum static tensile force of 1#steel is 17.6kN;and that is 16.5kN for 2#steel.