Control Of Microstructure And Mechanical Properties Of Low-carbon And Low-alloy High Strength Automobile Steel

Advanced high strength steel is easy to form stable surface oxides such as Mn2SiO4,when it contains too much Si,Mn and other alloying elements.These oxides not only reduce the surface quality of hot-rolled and cold-rolled steel,but also cut off zinc and steel plate reaction in the hot-dip galvanizing line.Considering that A1 has the similar function to Si,but they are more inclined to internal oxidation.Therefore this article according to the composition design idea of "the substitution of Al for Si" designed a new advanced high strength steel.Through different process design,the quenching and partitioning process and continuous annealing process of cold-rolled dual phase steel were studied respectively.The main works are present as follows:(1)According to the principle "the substitution of A1 for Si",about 0.8%Al(Wt.%)was added in experimental steel partial replacement of Si.Meanwhile,to make up for the loss of strength,some other alloy elements were added,such as P,Nb,Cr etc.The element P played a role of solid solution strengthening and element Nb played a role of precipitation strengthening and fine-grain strengthening.Fully considered the balance of various elements and combined with Thermo-Calc software,a new high strength steel was designed.Its final composition is:0.17C-0.6Si-0.8Mn-0.075P-0.82A1-0.048Nb-0.19Mo.(2)The equilibrium phase diagram of the experimental steel and the volume fraction of each phase at different temperatures were calculated by using Thermo-Calc.The continuous cooling transformation behavior of experimental steel was studied by dilatometer.The phase transition interval and critical cooling rate were acquired,and the static CCT curve was plotted.(3)The result of high temperature austenite deformation behavior shows that the dynamic recrystallization launch easier when the deformation temperature is higher and the strain rate is lower.The activation energy of dynamic recrystallization is 310.378kJ/mol.When the interval time is 5 s,the static recrystallization has be closed to the finished state at 1100℃.(4)The quenching and partitioning process of tested steel has been systematically studied.Compared with complete austenitizing annealing,the elongation of the samples increased significantly after intercritical annealing.The volume of retained austenite increases first and then decreases with the increase of annealing temperature.Through the CCE model calculation,the best quenching temperature of the tested steel is 264℃.The mechanical properties show that with the decrease of quenching temperature,the tensile strength and yield strength of the steel increase,and the elongation decreases.In the study of the partitioning process,it was found that with partitioning temperature increasing,the tensile strength decreased,yield strength and elongation increased.With the improvement of partitioning time,the volume of retained austenite increased first and then decreased.Finally the optimum heat treatment parameters of the tested steel were obtained,i.e,the annealing temperature was 967℃,the quenching temperature was 260℃ the partitioning temperature was 450℃ and the partitioning time was 300s.The optimized mechanical property could be obtained at the optimum quenching and partitioning process parameters.The tensile strength,elongation and product of strength and elongation reached 728MPa,30%and 21840 MPa·%,respectively.(5)The continuous annealing process of cold-rolled dual phase was simulated on the multi-function continuous annealing testing machine.The effects of different annealing temperature and slow cooling temperature on the microstructure and properties were studied.With the increase of annealing temperature the tensile strength increased first and the decreased,the yield strength increased gradually.With decreasing slow cooling temperature,the amount of epitaxial ferrite increased,tensile strength and yield strength decreased,elongation and product of strength and elongation increased.The best mechanical property was obtained when the annealing temperature was 800℃,the slow cooling temperature was 670℃.The tensile strength,elongation and product of strength and elongation reached 760MPa,24%and 18240 MPa·%,respectively.