The Study On Hot Deformation Behavior And γ�'α Transformation Behavior Of Fine Grain High-strength IF Steel

In order to reduce the weight of automobile, save the energy cost and improve the collision security, the high-strength steel must be used on automotive. The series of automotive plates is between 280MPa and 980MPa on abroad. The amount of high-strength plates were used on automobile increases year by year.The fine grain high-strength IF steel is got by refinement of the grain, increasing strength of the grain boundary, restraining the secondly working brittle, no adding the dissolution element which influencing the surface quality, using the precipitation strength by the finest particles, so it has excellent deep-drawing performance and low yield ratio. At present, fine grain high-strength IF(CR and GA)steel of 340MPa> 390MPa and 440MPa have been produced.In this paper, the hot deformation behavior of fine grain high-strength IF steel were studied on MMS-300 thermo-mechanical simulator. The main contents are given as follows:1. The single pass compression test showed that with10s^-1 strain rate and the amount of the true strain is 0.5, the mode of the stress-strain curve is dynamic recovery, no change with temperature. The flow stresses increase with increasing of temperature; When hot deformation at 900℃or 950℃, the flow stresses increase with the increasing of deformation rate; The flow stresses increase with increasing of reduction as temperature and strain rate are constant.2. According to true stress-strain curves of single-pass compression, the resistance mathematic models of fine grain high-strength IF steel were established:σ=1219.26 ? (ε)⁰.775(ε|·)^0.022 exp(-0.001T - 0.956ε)then calculated the austenite activation energy by Z-hollomen parameters:q =196.92kJ/mol.3. By double-pass compression test, the static softening behavior of fine grain high-strength IF steel was studied. The static softening rate increases with high deformation temperature and long holding time, the size of grain becomes big too; Through Z-hollomen parameters, the static rate was calculated, and the static recrystallization activation energy is 186.73kJ/mol, the results indicated that the recrystallization behavior of the steel in this paper could happened easily. 4. The CCT curve of fine grain high-strength IF steel was carried out; deformation condition and cooling rate have great effects on transformation point; deformation accelerates the ferrite transformation; the transition region of ferrite enlarges with increasing the cooling rate. The TEM pictures showed that dislocation density in ferrite grain increased with increasing the cooling rate, Nb(C,N) and NbC were precipitated in the grain.5. The optimum hot rolling process of fine grain high-strength IF steel was obtained from the reserved scheme: The heating temperature is 1100℃, the roughing temperature is 1050℃, the finishing temperature is 900℃and the coiling temperature is 640℃.